draft-ietf-mmusic-rtsp-nat-01.txt   draft-ietf-mmusic-rtsp-nat-02.txt 
Network Working Group Magnus Westerlund Network Working Group Magnus Westerlund
INTERNET-DRAFT Ericsson INTERNET-DRAFT Ericsson
Category: Standards Track Thomas Zeng Category: Standards Track Thomas Zeng
Expires: December 2003 PacketVideo Expires: August 2004 PacketVideo Network Solutions
June 30, 2003 Feb 16, 2004
How to Enable Real-Time Streaming Protocol (RTSP) traverse Network How to Enable Real-Time Streaming Protocol (RTSP) traverse Network
Address Translators (NAT) and interact with Firewalls. Address Translators (NAT) and interact with Firewalls.
<draft-ietf-mmusic-rtsp-nat-01.txt> <draft-ietf-mmusic-rtsp-nat-02.txt>
Status of this memo Status of this memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other Task Force (IETF), its areas, and its working groups. Note that other
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skipping to change at page 1, line 38 skipping to change at page 1, line 38
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The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
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This document is an individual submission to the IETF. Comments This document is an individual submission to the IETF. Comments
should be directed to the authors. should be directed to the authors.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract Abstract
This document describes six different types of NAT traversal This document describes six different types of NAT traversal
techniques that can be used by RTSP. For each technique a description techniques that can be used by RTSP. For each technique a description
on how it shall be used, what security implications it has and other on how it shall be used, what security implications it has and other
deployment considerations are given. Further a description on how deployment considerations are given. Further a description on how
RTSP relates to firewalls is given. RTSP relates to firewalls is given.
TABLE OF CONTENTS TABLE OF CONTENTS
1. Definitions.........................................................3 1. Definitions.........................................................3
1.1. Glossary.......................................................3 1.1. Glossary.......................................................3
1.2. Terminology....................................................3 1.2. Terminology....................................................3
2. Changes.............................................................3 2. Changes.............................................................3
3. Introduction........................................................4 3. Introduction........................................................4
3.1. NATs...........................................................5 3.1. NATs...........................................................4
3.2. Firewalls......................................................6 3.2. Firewalls......................................................5
4. Detecting the loss of NAT mappings..................................6 4. Requirements........................................................6
5. NAT Traversal Techniques............................................7 5. Detecting the loss of NAT mappings..................................6
5.1. STUN...........................................................7 6. NAT Traversal Techniques............................................7
5.1.1. Introduction..............................................7 6.1. STUN...........................................................8
5.1.2. Using STUN to traverse NAT without server modifications...8 6.1.1. Introduction..............................................8
5.1.3. Embedding STUN in RTSP...................................10 6.1.2. Using STUN to traverse NAT without server modifications...8
5.1.4. Discussion On Co-located STUN Server.....................13 6.1.3. Embedding STUN in RTSP...................................10
5.1.5. ALG considerations.......................................13 6.1.4. Discussion On Co-located STUN Server.....................11
5.1.6. Deployment Considerations................................13 6.1.5. ALG considerations.......................................12
5.1.7. Security Considerations..................................15 6.1.6. Deployment Considerations................................12
5.2. ICE...........................................................16 6.1.7. Security Considerations..................................13
5.2.1. Introduction.............................................16 6.2. ICE...........................................................14
5.2.2. Using ICE in RTSP........................................17 6.2.1. Introduction.............................................14
5.2.3. Required Protocol Extensions.............................18 6.2.2. Using ICE in RTSP........................................15
5.2.4. Implementation burden of ICE.............................18 156.2.3. Implementation burden of ICE...........................15
5.2.5. Deployment Considerations................................18 6.2.4. Deployment Considerations................................15
5.2.6. Security Considerations..................................19 6.2.5. Security Considerations..................................16
5.3. Symmetric RTP.................................................20 6.3. Symmetric RTP.................................................16
5.3.1. Introduction.............................................20 6.3.1. Introduction.............................................16
5.3.2. Necessary RTSP extensions................................20 166.3.2. Using Symmetric RTP in RTSP............................17
5.3.3. Using Symmetric RTP in RTSP..............................21 6.3.3. Open Issues..............................................17
5.3.4. Open Issues..............................................23 6.3.4. Deployment Considerations................................17
5.3.5. Deployment Considerations................................24 6.3.5. Security Consideration...................................17
5.3.6. Security Consideration...................................24 6.4. Application Level Gateways....................................18
5.4. Application Level Gateways....................................25 6.4.1. Introduction.............................................18
5.4.1. Introduction.............................................25 6.4.2. Guidelines On Writing ALGs for RTSP......................19
5.4.2. Guidelines On Writing ALGs for RTSP......................26 6.4.3. Deployment Considerations................................20
5.4.3. Deployment Considerations................................27 6.4.4. Security Considerations..................................20
5.4.4. Security Considerations..................................27 6.5. TCP Tunneling.................................................20
5.5. TCP Tunneling.................................................27 6.5.1. Introduction.............................................20
5.5.1. Introduction.............................................27 6.5.2. Usage of TCP tunneling in RTSP...........................21
5.5.2. Usage of TCP tunneling in RTSP...........................28 6.5.3. Deployment Considerations................................21
5.5.3. Deployment Considerations................................28 6.5.4. Security Considerations..................................21
5.5.4. Security Considerations..................................28 6.6. TURN (Traversal Using Relay NAT)..............................21
5.6. TURN (Traversal Using Relay NAT)..............................29 6.6.1. Introduction.............................................21
5.6.1. Introduction.............................................29 6.6.2. Usage of TURN with RTSP..................................22
5.6.2. Usage of TURN with RTSP..................................29 6.6.3. Deployment Considerations................................23
5.6.3. Deployment Considerations................................30 6.6.4. Security Considerations..................................23
5.6.4. Security Considerations..................................31 7. Firewalls..........................................................24
6. Firewalls..........................................................31
7. Open Issues........................................................32 8. Open Issues........................................................25
8. Security Consideration.............................................32 9. Security Consideration.............................................25
9. IANA Consideration.................................................33 10. IANA Consideration................................................26
10. Acknowledgments...................................................33 11. Acknowledgments...................................................26
11. Author's Addresses................................................33 12. Author's Addresses................................................26
12. References........................................................35 13. References........................................................27
12.1. Normative references.........................................35 13.1. Normative references.........................................27
12.2. Informative References.......................................35 13.2. Informative References.......................................27
13. IPR Notice........................................................36 14. IPR Notice........................................................28
14. Copyright Notice..................................................36 15. Copyright Notice..................................................29
1. Definitions 1. Definitions
1.1. Glossary 1.1. Glossary
ALG Application Level Gateway, an entity that can be embedded in ALG û Application Level Gateway, an entity that can be embedded in
a NAT to perform the application layer functions required a NAT to perform the application layer functions required
for a particular protocol to traverse the NAT [6] for a particular protocol to traverse the NAT [6]
ICE - Interactive Connectivity Establishment, see [9]. ICE - Interactive Connectivity Establishment, see [9].
DNS Domain Name Service DNS û Domain Name Service
MID - Media Identifier from Grouping of media lines in SDP, see MID - Media Identifier from Grouping of media lines in SDP, see
[10]. [10].
NAT - Network Address Translator, see [12]. NAT - Network Address Translator, see [12].
NAT-PT - Network Address Translator Protocol Translator, see [13] NAT-PT - Network Address Translator Protocol Translator, see [13]
RTP - Real-time Transport Protocol, see [5]. RTP - Real-time Transport Protocol, see [5].
RTSP - Real-Time Streaming Protocol, see [1] and [7]. RTSP - Real-Time Streaming Protocol, see [1] and [7].
SDP - Session Description Protocol, see [2]. SDP - Session Description Protocol, see [2].
SSRC - Synchronization source in RTP, see [5]. SSRC - Synchronization source in RTP, see [5].
TBD - To Be Decided
1.2. Terminology 1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4]. document are to be interpreted as described in RFC 2119 [4].
2. Changes 2. Changes
The following changes has been done since draft-ietf-mmusic-rtsp-nat- The following changes has been done since draft-ietf-mmusic-rtsp-nat-
00.txt: 00.txt:
- New co-author Thomas Zeng. - Added requirements section, per discussions during IETF 58.
- Added a chapter on the usage of ICE in RTSP. - Delegated the discussion on using ICE for RTSP to a separate
- Added a definition for how to use STUN embedded to traverse draft.
symmetric NATs. - Removed all the solutions proposal in regards protocol changing
- Added chapter on detecting loss of NAT mappings. mechanism.
- More text on Firewalls.
- Symmetric RTP description has been extended with use case with a
few well-known ports on the server side.
- Added text on transition strategies for the methods.
- Improved language in the whole draft.
- An Open Issues section has been created.
3. Introduction 3. Introduction
Today there is a proliferative deployment of different flavors of Today there is a proliferative deployment of different flavors of
Network Address Translator (NAT) boxes that in practice follow no Network Address Translator (NAT) boxes that in practice follow no
open standards [12][18]. NATs cause discontinuity in address realms open standards [12][18]. NATs cause discontinuity in address realms
[18], therefore a protocol, such as RTSP, needs to try to make sure [18], therefore a protocol, such as RTSP, needs to try to make sure
that it can deal with such discontinuities caused by NATs. The that it can deal with such discontinuities caused by NATs. The
problem with RTSP is that, being a media control protocol that problem with RTSP is that, being a media control protocol that
manages one or more media streams, it carries information about manages one or more media streams, it carries information about
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considered. They are deployed to prevent non-desired considered. They are deployed to prevent non-desired
traffic/protocols to be able to get in or out of the protected traffic/protocols to be able to get in or out of the protected
network. RTSP is designed such that a firewall can be configured to network. RTSP is designed such that a firewall can be configured to
let RTSP controlled media streams to go through with minimal let RTSP controlled media streams to go through with minimal
implementation problems. However there is a need for more detailed implementation problems. However there is a need for more detailed
information on how FWs should be configured to work with RTSP. information on how FWs should be configured to work with RTSP.
This document describes the usage of known NAT traversal mechanisms This document describes the usage of known NAT traversal mechanisms
that can be used with RTSP. Following the guidelines spelled out in that can be used with RTSP. Following the guidelines spelled out in
[18], we describe the required RTSP protocol extensions for each [18], we describe the required RTSP protocol extensions for each
method, transition strategies, and we also discuss each methods method, transition strategies, and we also discuss each methodÆs
security concerns. security concerns.
Some of the NAT/FW traversal solutions are based on IETF internet
drafts in their early stage of standardization (e.g., ICE and TURN).
Given the current demand for NAT traversal solutions in the RTSP
market place, it is foreseeable that a standard be created or
adopted, in a timely fashion, by IETF MMUSIC WG to solve NAT
traversal problem specifically for RTSP based streaming systems.
This document is not based on RFC 2326 [1]. It is instead based and This document is not based on RFC 2326 [1]. It is instead based and
dependent on the updated RTSP specification [7], which is under dependent on the updated RTSP specification [7], which is under
development in IETF MMUSIC WG. The updated specification is a much- development in IETF MMUSIC WG. The updated specification is a much-
needed attempt to correct a number of shortcomings of RFC 2326. One needed attempt to correct a number of shortcomings of RFC 2326. One
important change is that the specification is split into several important change is that the specification is split into several
parts. So far only the updated core specification of RTSP is parts. So far only the updated core specification of RTSP is
available in [7]. This document is one extension document to this available in [7]. This document is one extension document to this
core spec to document a special functionality that extends the RTSP core spec to document a special functionality that extends the RTSP
protocol. This document is intended to be updated to stay consistent protocol. This document is intended to be updated to stay consistent
with the core protocol. with the core protocol.
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control policies can be implemented using address translation control policies can be implemented using address translation
schemes. schemes.
3. NAT and FWs are similar in that they can both be configured to 3. NAT and FWs are similar in that they can both be configured to
allow multiple network hosts to share a single public IP address. allow multiple network hosts to share a single public IP address.
In other words, a host behind a NAT or FW can have a private IP In other words, a host behind a NAT or FW can have a private IP
address and a public one, so for NAT and FW there is the issue of address and a public one, so for NAT and FW there is the issue of
address mapping which is important in order for RTSP protocol to address mapping which is important in order for RTSP protocol to
work properly when there are NATs and FWs between the RTSP server work properly when there are NATs and FWs between the RTSP server
and its clients. and its clients.
4. Detecting the loss of NAT mappings In the rest of this memo we use the phrase ôNAT traversalö
interchangeably with ôNAT/FW traversalö and ôNAT/Firewall traversalö.
Several of the described NAT traversal techniques in the next chapter 4. Requirements
use the fact that the NAT UDP mapping's external address and port can
be discovered. This information is then utilized to direct the This section considers the set of requirements when designing or
traffic intended for the local side's address to the external evaluating RTSP NAT solutions.
instead. However any such information is only valid while the
mapping is intact. As the IAB's UNSAF document [18] points out the RTSP is a client/server protocol, and as such the targeted
mapping can either timeout or change its properties. It is therefore applications in general deploy RTSP servers in the public address
important for the NAT/FW traversal solutions to handle the loss or realm. However, there are use cases where the reverse is true: RTSP
change of NAT mappings, according to UNSAF. clients are connecting from public address realm to RTSP servers
behind home NATs. This is the case for instance when home
surveillance cameras running as RTSP servers intend to stream video
to cell phone users in the public address realm through a home NAT.
The first priority should be to solve the RTSP NAT traversal problem
for RTSP servers deployed in the open.
The list of feature requirements for RTSP NAT solutions are given
below:
1. MUST work for all flavors of NATs, including symmetric NATs
2. MUST work for firewalls (subject to pertinent firewall
administrative policies), including those with ALGs
3. SHOULD have minimal impact on clients in the open and not dual-
hosted
o For instance, no extra delay from RTSP connection till
arrival of media
4. SHOULD be simple to use/implement/administer that people
actually turn them on
o Otherwise people will resort to TCP tunneling through NATs
o Address discovery for NAT traversal should take place
behind the scene, if possible
5. SHOULD authenticate dual-hosted client transport handler to
prevent DDOS attacks
5. Detecting the loss of NAT mappings
Several of the NAT traversal techniques in the next chapter use the
fact that the NAT UDP mapping's external address and port can be
discovered. This information is then utilized to direct the traffic
intended for the local side's address to the external instead.
However any such information is only valid while the mapping is
intact. As the IAB's UNSAF document [18] points out, the mapping can
either timeout or change its properties. It is therefore important
for the NAT traversal solutions to handle the loss or change of NAT
mappings, according to UNSAF.
First, it is important to ensure that there exists the possibility to First, it is important to ensure that there exists the possibility to
send keep-alive traffic to minimize the probability of timeout. The send keep-alive traffic to minimize the probability of timeout. The
difficulty is that the timeout timer can have varying length between difficulty is that the timeout timer can have varying length between
different NATs. That is the reason why that UNSAF recommends usage of different NATs. That is the reason why that UNSAF recommends usage of
STUN to determine this timeout. STUN to determine this timeout.
Secondly, it is possible to detect and recover from the situation Secondly, it is possible to detect and recover from the situation
where the mapping has been changed or removed. The possibility to where the mapping has been changed or removed. The possibility to
detect a lost mapping is based on the fact that no traffic will detect a lost mapping is based on the fact that no traffic will
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The loss of mapping carrying RTCP is simpler to detect. As RTCP is The loss of mapping carrying RTCP is simpler to detect. As RTCP is
normally sent periodically in each direction, even during the RTSP normally sent periodically in each direction, even during the RTSP
ready state, if RTCP packets are missing for several RTCP intervals, ready state, if RTCP packets are missing for several RTCP intervals,
the mapping is likely to be lost. Note that if no RTCP packets are the mapping is likely to be lost. Note that if no RTCP packets are
received by the RTSP server for a while, the RTSP server has the received by the RTSP server for a while, the RTSP server has the
option to delete the corresponding SSRC and RTSP session ID, which option to delete the corresponding SSRC and RTSP session ID, which
means either the client could not get through a middle box NAT/FW, or means either the client could not get through a middle box NAT/FW, or
that the client is mal-functioning. that the client is mal-functioning.
5. NAT Traversal Techniques 6. NAT Traversal Techniques
There exist a number of NAT traversal techniques that can be used to There exist a number of NAT traversal techniques that can be used to
allow RTSP to traverse NATs. However they have different features, allow RTSP to traverse NATs. However they have different features,
they are applicable to different topologies; and the cost is also they are applicable to different topologies; and the cost is also
different. They also differ in their security considerations. In the different. They also differ in their security considerations. In the
following sections, each technique is outlined in details in terms of following sections, each technique is outlined in details in terms of
its advantages and disadvantages. its advantages and disadvantages.
Not all of the techniques are yet described in the full details Not all of the techniques are yet described in the full details
needed to actually use this document as a specification for how to needed to actually use this document as a specification for how to
use them. These sections are included to present comparison amongst use them. These sections are included to present comparison amongst
the different methods in order for one to identify the most suitable the different methods in order for one to identify the most suitable
method for a particular RTSP deployment scenario. There are methods method for a particular RTSP deployment scenario. There are methods
that use protocols in early stage of standardization, such as TURN that use protocols in early stage of standardization, such as TURN
and ICE. and ICE.
5.1. STUN 6.1. STUN
5.1.1. Introduction 6.1.1. Introduction
STUN – Simple Traversal of UDP Through Network Address Translators STUN û ôSimple Traversal of UDP Through Network Address Translatorsö
[6] is a standardized protocol developed by the MIDCOM WG that allows [6] is a standardized protocol developed by the MIDCOM WG that allows
a client to use secure means to discover the presence of a NAT a client to use secure means to discover the presence of a NAT
between himself and the STUN server and the type of that NAT. The between himself and the STUN server and the type of that NAT. The
client then uses the STUN server to discover the address bindings client then uses the STUN server to discover the address bindings
assigned by the NAT. The protocol also allows discovery of the assigned by the NAT. The protocol also allows discovery of the
mappings timeout period and can be used in any keep-alive mechanism. mappings timeout period and can be used in any keep-alive mechanism.
STUN is a client-server protocol. STUN client sends a request to a STUN is a client-server protocol. STUN client sends a request to a
STUN server and the server returns a response. There are two types of STUN server and the server returns a response. There are two types of
STUN requests Binding Requests, sent over UDP, and Shared Secret STUN requests û Binding Requests, sent over UDP, and Shared Secret
Requests, sent over TLS over TCP. We note here that for RTSP clients Requests, sent over TLS over TCP. We note here that for RTSP clients
running on embedded devices, it may not be practical to require TLS running on embedded devices, it may not be practical to require TLS
be implemented on the embedded device (such as a cell phone). be implemented on the embedded device (such as a cell phone).
Therefore in the next section we propose to adapt RFC 3489 ([6]) so Therefore in the next section we propose to adapt RFC 3489 ([6]) so
as to let RTSP use a subset of STUN packets/features for NAT as to let RTSP use a subset of STUN packets/features for NAT
traversal, but without requiring full implementation of STUN in an traversal, but without requiring full implementation of STUN in an
RTSP server or RTSP client. We note that RFC 3489 has provisions for RTSP server or RTSP client. We note that RFC 3489 has provisions for
STUN to be embedded in another application (see section 6 of [6]). STUN to be embedded in another application (see section 6 of [6]).
5.1.2. Using STUN to traverse NAT without server modifications 6.1.2. 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. However this RTSP servers without requiring server modifications. However 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:
- The server must be located in either a public address realm or the - The server must be located in either a public address realm or the
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mapping, UDP packets needs to be sent first to the servers source mapping, UDP packets needs to be sent first to the servers source
address/port. To minimize potential effects on the server from these address/port. To minimize potential effects on the server from these
messages the following type of messages MUST be sent. RTP: An empty messages the following type of messages MUST be sent. RTP: An empty
or less than 12 bytes large UDP message. RTCP: A correctly formed or less than 12 bytes large UDP message. RTCP: A correctly formed
RTCP message. RTCP message.
The above described adaptations for restricted NATs will not work The above described adaptations for restricted NATs will not work
unless the server includes the "src_addr" "Transport" header unless the server includes the "src_addr" "Transport" header
parameter. parameter.
5.1.3. Embedding STUN in RTSP 6.1.3. Embedding STUN in RTSP
This section describes 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 adaptation is an extension to the core including symmetric NATs. Any protocol changes are beyond the scope
RTSP protocol [7], and therefore is signaled by feature tag. As of this memo and is instead defined in TBD internet draft.
specified in [7], features are recommended to be negotiated using
"supported" headers.
We define the feature tag for embedded STUN with out authentication
support as:
nat.stun
and for embedded STUN supporting authentication as:
nat.stun-auth
If one side supports "nat.stun-auth" but the other side only supports
"nat.stun", then both sides must go through negotiation and possibly
downgrade to using "nat.stun". If one RTSP end system refuses to
accept "nat.stun", then do not use STUN for RTSP.
Limitations: Limitations:
This NAT traversal solution (using STUN with RTSP) has limitations: This NAT traversal solution (using STUN with RTSP) 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. In the case of "nat.stun", the RTSP server may, for security 2. The RTSP server may, for security reasons, refuse to send media
reasons, refuse to send media streams to an IP different from streams to an IP different from the IP in the client RTSP
the IP in the client RTSP requests. Therefore, if the client is requests. Therefore, if the client is behind a NAT that has
behind a NAT that has multiple public addresses, and the multiple public addresses, and the clientÆs RTSP port and UDP
client’s RTSP port and UDP port are mapped to different IP port are mapped to different IP addresses, RTSP SETUP will
addresses, RTSP SETUP will fail. fail.
Deviations from STUN as defined in RFC2389 Deviations from STUN as defined in RFC 3489
Specifically, we differ from RFC3489 in two aspects: Specifically, we differ from RFC3489 in two aspects:
1. We allow RTSP applications to have the option to perform 1. We allow RTSP applications to have the option to perform
"binding discovery" without authentication; "binding discovery" without authentication;
2. We require STUN server be co-located on RTSP servers media 2. We require STUN server be co-located on RTSP serverÆs media
ports. 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 would ignore authentication tag, server embedded in RTSP application would ignore authentication tag,
and the STUN client embedded in RTSP application would use dummy and the STUN client embedded in RTSP application would use dummy
authentication tag, as well. authentication tag, as well.
In order to use STUN to solve NAT traversal when RTSP client is In order to use STUN to solve NAT traversal when RTSP client is
behind a symmetric NAT, STUN server must co-locate on RTSP servers behind a symmetric NAT, STUN server must co-locate on RTSP serverÆs
media ports. This can be done, for instance, by embedding STUN server media ports. This can be done, for instance, by embedding STUN server
in RTSP server. in RTSP server.
In fact, if STUN server is indeed co-located with RTSP servers media In fact, if STUN server is indeed co-located with RTSP serverÆs media
port, then a RTSP client using RTP transport over UDP can use STUN to port, then a RTSP client using RTP transport over UDP can use STUN to
traverse ALL types of NATs that have been defined in section 3.1. In traverse ALL types of NATs that have been defined in section 3.1. In
the case of symmetric NAT, the party inside the NAT must initiate UDP the case of symmetric NAT, the party inside the NAT must initiate UDP
traffic. The STUN Bind Request, being a UDP packet itself, can serve traffic. The STUN Bind Request, being a UDP packet itself, can serve
as the traffic initiating packet. Subsequently, both the STUN Binding as the traffic initiating packet. Subsequently, both the STUN Binding
Response packets and the RTP/RTCP packets can traverse the NAT, Response packets and the RTP/RTCP packets can traverse the NAT,
regardless of whether the RTSP server or the RTSP client is behind regardless of whether the RTSP server or the RTSP client is behind
NAT. NAT.
Likewise, if a RTSP server is behind a NAT, then an embedded STUN Likewise, if a RTSP server is behind a NAT, then an embedded STUN
server must co-locate on the RTSP clients RTCP port. In this case, server must co-locate on the RTSP clientÆs RTCP port. In this case,
we assume that the client has some means to establish TCP connection we assume that the client has some means to establish TCP connection
to the RTSP server behind NAT so as to exchange RTSP messages with to the RTSP server behind NAT so as to exchange RTSP messages with
the RTSP server. the RTSP server.
RTSP implementations supporting such features must use the feature
tag, (nat.stun-auth or nat.stun) to indicate to each other the
availability of such embedded, co-located STUN servers.
To minimize delay, we require that the RTSP server supporting this To minimize delay, we require that the RTSP server supporting this
option must inform its client the RTP and RTCP ports that the server option must inform its client the RTP and RTCP ports that the server
intend to send RTP and RTCP packets, respectively. intend to send RTP and RTCP packets, respectively.
To minimize the keep-alive traffic for address mapping, we also To minimize the keep-alive traffic for address mapping, we also
require that the RTSP end-point (server or client) sends and receives require that the RTSP end-point (server or client) sends and receives
RTCP packets from the same port. RTCP packets from the same port.
RTSP NAT Traversal Algorithm Using STUN 6.1.4. Discussion On Co-located STUN Server
The actual NAT traversal algorithm contains six steps.
Step 1:
This first step is for both RTSP server and client to
discover whether there is a NAT, and if yes, the timeout
period for UDP mapping on the NAT. For the RTSP client, as
soon as it has learnt that the RTSP server supports the
"nat.stun" or "nat.stun-auth" feature, and that it has learnt
the RTSP server’s RTP and RTCP ports, it should send STUN
request packets to those ports, and also include the
appropriate feature tag (either nat.stun or nat.stun-auth) in
all of its relevant RTSP requests and responses.
On the other hand, a RTSP server can figure out whether it is
in the public Internet at start up time. If it turns out that
the RTSP server is in a private address realm, the RTSP server
must be prepared to receive STUN Binding Request on its RTCP
receive port (so as to help RTSP client’s RTCP RR reports to
reach the right destination). Otherwise, if it turns out that
RTSP server is in the public address realm, it must be
prepared to do the following:
- If "nat.stun" is the agreed-upon feature tag between server
and client, the RTSP server must monitor its RTP and RTCP
send ports for STUN Binding Requests;
- If "nat.stun-auth" is the agreed-upon feature tag between
server and client, the RTSP server must monitor its RTP and
RTCP send ports for STUN Shared Secrete Requests and Binding
Requests;
Step 2:
The RTSP client determines the number of UDP ports needed by
counting the number of RTP sessions in the multi-media
presentation. This information is available in the media
description protocol, e.g. SDP [2], and according to the
client’s media selection criteria. In general each RTP session
will require two UDP ports, one for RTP, and one for RTCP. The
RTSP client also obtains, for each RTP session, the media port
from which RTSP server will send out the RTP packets.
Step 3:
This step applies if the client knows, from step 1, that it
is behind NAT. For each UDP port required, the RTSP client
must open a local socket using an available UDP port on the
host computer, establish a mapping and discover the public IP
address and port number with the help of the STUN server co-
located at the RTSP server’s media ports. Assume STUN
protocol exchange is successful, an address mapping will be
sent back to the RTSP client in a STUN response packet, then
the RTSP client must record the mapping between client’s local
address/port and the external address/port in its database.
Step 4:
RTSP client then performs the RTSP SETUP for each media. In
the transport header the following parameter SHOULD be
included with the given values: "dest_addr" with the external
IP address and port pair for both RTP and RTCP. To allow this
to work servers MUST allow a client to setup the RTP stream on
any port, not only even ports.
Step 5:
This step only applies if client is in the open, but RTSP
server discovers, with the help of a public STUN server, that
it is the one behind NAT. RTSP server obtains the client’s
RTCP port number from the SETUP request, and immediately sends
STUN request to that port to obtain the address mapping.
Assume again the mapping is obtained successfully, then the
server SHALL respond with a transport header containing a
"src_addr" parameter with the mapped RTCP source IP address
and port.
Step 6:
To keep the mappings alive, the party that is behind NAT
SHOULD periodically send UDP traffic over all mappings needed
for the session when no traffic is received. For the mapping
carrying RTCP traffic the periodic RTCP traffic may be enough.
For mappings carrying RTP traffic and for mappings carrying
RTCP traffic infrequently, keep alive messages SHOULD be sent.
STUN packets can serve as keep alive messages, given the
requirement to have STUN server collocates on the RTSP
server’s media ports.
If a UDP mapping is lost then the above discovery process is required
to be performed again. The media stream needs to be SETUP again to
change the transport parameters to the new ones. This will likely
cause a glitch in media playback.
5.1.4. Discussion On Co-located STUN Server
In order to use STUN to traverse symmetric NATs the STUN server needs In order to use STUN to traverse symmetric NATs the STUN server needs
to be co-located with the streaming server media \ports, i.e., the to be co-located with the streaming server media ports, i.e., the
port from which RTP packets will be sent. This creates a de- port from which RTP packets will be sent. 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. This from a media packet. This will be done based on heuristics. This
works fine between STUN and RTP or RTCP where the first byte has works fine between STUN and RTP or RTCP where the first byte has
always present difference, but this can't be guaranteed to work with always present difference, but this can't be guaranteed to work with
other media protocols. other media protocols.
5.1.5. ALG considerations 6.1.5. 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. resulting in unpredictable behavior.
5.1.6. Deployment Considerations 6.1.6. Deployment Considerations
For the non-embedded usage of STUN the following applies: For the non-embedded usage of STUN the following applies:
Advantages: Advantages:
- Using STUN does not require RTSP server modifications, it only - Using STUN does not require RTSP server modifications; it only
affects the client implementation. affects the client implementation.
Disadvantages: Disadvantages:
- Requires a STUN server deployed in the public address space. - Requires a STUN server deployed in the public address space.
- Only works with Cone NATs. Restricted Cone NATs create some - Only works with Cone NATs. Restricted Cone NATs create some
issues. Does not work with Symmetric NATs without server issues. Does not work with Symmetric NATs without server
modifications. modifications.
- Will mostly not work if a NAT uses multiple IP addresses, since - Will mostly not work if a NAT uses multiple IP addresses, since
RTSP server generally requires all media streams to use the same IP RTSP server generally requires all media streams to use the same IP
skipping to change at page 14, line 41 skipping to change at page 13, line 4
need at all exists to use STUN. need at all exists to use STUN.
For the Embedded STUN method the following applies: For the Embedded STUN method the following applies:
Advantages: Advantages:
- STUN is a solution first used by SIP applications. As shown above, - 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 with little or no changes, RTSP application can re-use STUN as a
NAT traversal solution, avoiding the pit-fall of solving a problem NAT traversal solution, avoiding the pit-fall of solving a problem
twice. twice.
- STUN has built-in message authentication features, which makes it - STUN has built-in message authentication features, which makes it
more secure. See next section for an in-depth security discussion. more secure. See next section for an in-depth security discussion.
- This solution works as long as there is only one RTSP end point in - This solution works as long as there is only one RTSP end point in
the private address realm, regardless of the NATs type. There may the private address realm, regardless of the NATÆs type. There may
even be multiple NATs (see figure 1 in [6]). even be multiple NATs (see figure 1 in [6]).
- Compares to other UDP based NAT traversal methods in this - Compares to other UDP based NAT traversal methods in this
document, STUN requires little new protocol development (since STUN document, STUN requires little new protocol development (since STUN
is already a IETF standard), and most likely less implementation is already a IETF standard), and most likely less implementation
effort, since open source STUN server and client have become effort, since open source STUN server and client have become
available [21]. There is the need to embed STUN in RTSP server and available [21]. There is the need to embed STUN in RTSP server and
client, which require a de-multiplexer between STUN packets and client, which require a de-multiplexer between STUN packets and
RTP/RTCP packets. There is also a need to register the proper RTP/RTCP packets. There is also a need to register the proper
feature tags. feature tags.
Disadvantages: Disadvantages:
- Feature tags must be registered with IANA. - Some extensions to the RTSP core protocol, signaled by RTSP
feature tags, must be introduced.
- Requires an embedded STUN server to co-locate on each of RTSP - Requires an embedded STUN server to co-locate on each of RTSP
servers media protocol's ports (e.g. RTP and RTCP ports), which serverÆs media protocol's ports (e.g. RTP and RTCP ports), which
means more processing is required to de-multiplex STUN packets from means more processing is required to de-multiplex STUN packets from
media packets. For example, the de-multiplexer must be able to media packets. For example, the de-multiplexer must be able to
differentiate a RTCP RR packet from a STUN packet, and forward the differentiate a RTCP RR packet from a STUN packet, and forward the
former to the streaming server, the later to STUN server. former to the streaming server, the later to STUN server.
- It does not work if none of the RTSP server and client is in the - It does not work if none of the RTSP server and client is in the
public address realm, and each of them is behind a different NAT. public address realm, and each of them is behind a different NAT.
- Even if the RTSP server is in the open, and the client is behind a - Even if the RTSP server is in the open, and the client is behind a
multi-addressed NAT, it may still break if the RTSP server does not multi-addressed NAT, it may still break if the RTSP server does not
allow RTP packets to be sent to an IP differs from the IP of the allow RTP packets to be sent to an IP differs from the IP of the
clients RTSP request. clientÆs RTSP request.
- Interaction problems exist when a RTSP ALG is not aware of STUN. - Interaction problems exist when a RTSP ALG is not aware of STUN.
- Using STUN requires that RTSP servers and clients support the - Using STUN requires that RTSP servers and clients support the
updated RTSP specification, and they both agree to support the updated RTSP specification, and they both agree to support the
proper feature tag. proper feature tag.
Transition: Transition:
The usage of STUN can be phased out gradually as the first step of a The usage of STUN can be phased out gradually as the first step of a
STUN capable machine can be to check the presence of NATs for the STUN capable machine can be to check the presence of NATs for the
presently used network connection. The removal of STUN capability in presently used network connection. The removal of STUN capability in
the client implementations will however most probably wait until the client implementations will however most probably wait until
there is no need at all to use STUN. When there is no more need to there is no need at all to use STUN.
use STUN, the feature tags, "nat.stun" and "nat.stun-auth", can be
de-registered at IANA.
5.1.7. Security Considerations 6.1.7. 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 server other than the one that RTSP message originates from. The RTSP server
is only prepared to make an exception of this rule when the client is is only prepared to make an exception of this rule when the client is
trusted (e.g., through the use of a secure authentication process, or trusted (e.g., through the use of a secure authentication process, or
through some secure method of challenging the destination to verify through some secure method of challenging the destination to verify
its willingness to accept the UDP traffic). Such restriction means its willingness to accept the UDP traffic). Such restriction means
that STUN does not work for NATs that would assign different IP that STUN does not work for NATs that would assign different IP
addresses to different UDP flows on its public side. Therefore most addresses to different UDP flows on its public side. Therefore most
multi-addressed NATs will not work with STUN. multi-addressed NATs will not work with STUN.
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. It restricted RTSP has the same security properties as the core RTSP. It
is protected from being used as a DoS attack tool unless the attacker is protected from being used as a DoS attack tool unless the attacker
has ability to hijack RTSP stream. has ability to hijack RTSP stream.
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 can't be used to protect servers against being DoS these mechanisms cannot securely prevent RTSP servers being used as
attack tools. DoS attack tools.
5.2. ICE 6.2. ICE
5.2.1. Introduction 6.2.1. Introduction
ICE (Interactive Connectivity Establishment) [9] is a methodology for ICE (Interactive Connectivity Establishment) [9] is a methodology for
NAT traversal that is under development for SIP. The basic idea is to NAT traversal that is under development for SIP. The basic idea is to
try, in a parallel fashion, all possible connection addresses that an try, in a parallel fashion, all possible connection addresses that an
end point may have. This allows the end-point to use the best end point may have. This allows the end-point to use the best
available UDP "connection" (meaning two UDP end-points capable of available UDP "connection" (meaning two UDP end-points capable of
reaching each other). The methodology has very nice properties in reaching each other). The methodology has very nice properties in
that basically all NAT topologies are possible to traverse. that basically all NAT topologies are possible to traverse.
Here is how ICE works. End point A collects all possible address that Here is how ICE works. End point A collects all possible address that
can be used, including local IP addresses, STUN derived addresses, can be used, including local IP addresses, STUN derived addresses,
TURN addresses. On each local port that any of these address and port TURN addresses. On each local port that any of these address and port
pairs leads to, a STUN server is installed. This STUN server only pairs leads to, a STUN server is installed. This STUN server only
accepts STUN requests using the correct authentication through the accepts STUN requests using the correct authentication through the
use of username and password. use of username and password.
End-point A then sends a request to establish connectivity with end- End-point A then sends a request to establish connectivity with end-
point B, which includes all possible ways to get the media through to point B, which includes all possible ways to get the media through to
A. Note that each of As published address/port pairs has a STUN A. Note that each of AÆs published address/port pairs has a STUN
server co-located. B, before responding to A, uses a STUN client to server co-located. B, before responding to A, uses a STUN client to
try to reach all the address and port pairs specified by A. The try to reach all the address and port pairs specified by A. The
destinations for which the STUN requests have successfully completed destinations for which the STUN requests have successfully completed
are then indicated. If bi-directional communication is intended the are then indicated. If bi-directional communication is intended the
end-point B must then in its turn offer A all its reachable address end-point B must then in its turn offer A all its reachable address
and port pairs, which then are tested by A. and port pairs, which then are tested by A.
If B fails to get any STUN response from A, all hope is not lost. If B fails to get any STUN response from A, all hope is not lost.
Certain NAT topologies require multiple tries from both ends before Certain NAT topologies require multiple tries from both ends before
successful connectivity is accomplished. The STUN requests may also successful connectivity is accomplished. The STUN requests may also
result in that more connectivity alternatives are discovered and result in that more connectivity alternatives are discovered and
conveyed in the STUN responses. conveyed in the STUN responses.
This chapter is not yet a full technical solution. It is mostly a This chapter is not yet a full technical solution. It is mostly a
feasibility study on how ICE could be applied to RTSP and what feasibility study on how ICE could be applied to RTSP and what
properties it would have. One nice thing about ICE for RTSP is that properties it would have. One nice thing about ICE for RTSP is that
it does make it possible to deploy RTSP server behind NAT/FIRWALL, a it does make it possible to deploy RTSP server behind NAT/FIRWALL, a
desirable option to some RTSP applications. desirable option to some RTSP applications.
5.2.2. Using ICE in RTSP 6.2.2. Using ICE in RTSP
The usage of ICE for RTSP requires that both client and server be The usage of ICE for RTSP requires that both client and server be
updated to include the ICE functionality. If both parties implement updated to include the ICE functionality. If both parties implement
the necessary functionality the following step-by-step algorithm the necessary functionality the following step-by-step algorithm
could be used to accomplish connectivity for the UDP traffic. could be used to accomplish connectivity for the UDP traffic.
This assumes that it is possible to establish a TCP connection for This assumes that it is possible to establish a TCP connection for
the RTSP messages between the client and the server. This is not the RTSP messages between the client and the server. This is not
trivial in scenarios where the server is located behind a NAT, and trivial in scenarios where the server is located behind a NAT, and
may require some TCP ports been opened, or the deployment of proxies, may require some TCP ports been opened, or the deployment of proxies,
etc. etc.
1. The client retrieves the SDP from the ICE enabled RTSP server. Refer to [22] for the mapping of ICE to RTSP.
This SDP contains indication that the RTSP server supports ICE and
gives the address/ports for each media and its necessary UDP streams.
This may require a SDP extension or possibly the "c=" lines in which
port numbers can be used. This will however require the server to
send media streams from well-known ports. This will result in that
many sessions will go over the same ports for servers handling
multiple users.
2. The client analyzes the SDP and determines the number of local UDP
ports it will need. For each port it also installs a STUN server with
authentication requirement using an authentication tag. From these
ports the client then tries a STUN request to the server's announced
ports, which are intercepted by the co-located STUN servers. If
successful, the client’s NAT bindings, as seen by the RTSP server,
are discovered by these STUN servers and sent back to the RTSP
client. Also, other addresses, including addresses from public STUN
servers and TURN addresses, can be collected by the RTSP client.
3. Client creates a SETUP request where he includes a number of
transport header specifications. The client may offer more than one
transport configurations, but for each configuration it will need to
create multiple specifications of destination addresses that it has
learned in descending priority order. The client also includes in the
transport specification the ICE indicator carrying the user name and
password required by the client's STUN servers.
4. The server receives the SETUP request and selects which transport
specification it would like to accept. Here all specifications are
the same except for destination address/port. For all specifications
in the configuration the server tries to "STUN" these
addresses/ports. Depending on the answer, the following results may
happen:
A. The RTSP server successfully connects to the client’s STUN
server, and the RTSP server selects the specification with
highest priority that yields a successful response and include
that address/port in the SETUP response's transport headers
destination field. The media is ready to be played.
B. The server fails to reach any of the client’s STUN servers. It
uses a new error code to inform the client of this. At the same
time it includes an updated SDP, which contains all addresses
that it is reachable on. The server might have learned some new
reachable addresses since the initial SDP. The client then tries
again by going to step 2 above and repeat the entire process. If
it fails multiple times the server and client eventually give up.
5.2.3. Required Protocol Extensions
1. A SDP extension to indicates that the server supports ICE. It
will also require that grouping of media lines [10] with the
alternative semantics [11] be used in the SDP to indicate the
different alternatives.
2. A new Transport header parameter that indicates that ICE shall be
used on these streams and a way to convey the authentication user
name and password that the server shall use to contact the
client’s STUN server.
3. A RTSP error code for failed ICE setup. That error code will also
need to include entity body in the response to carry the updated
SDP description.
5.2.4. Implementation burden of ICE 6.2.3. 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.
A RTSP server implementation requirements: Some RTSP server implementation requirements are:
- Full STUN server features - Full STUN server features
- limited STUN client features - limited STUN client features
- SDP extensions that includes MID [10] and ICE features
- Dynamic SDP generation with more parameters. - Dynamic SDP generation with more parameters.
- RTSP error code for ICE extension - RTSP error code for ICE extension
Client: Some client implantation requirements are:
- Limited STUN server features - Limited STUN server features
- Limited STUN client features - Limited STUN client features
- SDP extensions that include MID [10] and ICE features
- RTSP error code and ICE extension - RTSP error code and ICE extension
5.2.5. Deployment Considerations 6.2.4. Deployment Considerations
Advantages:
Advantages:
- Solves NAT connectivity discovery for basically all cases as long - Solves NAT connectivity discovery for basically all cases as long
as a TCP connection between them can be established in the first as a TCP connection between them can be established in the first
hand. This includes servers behind NATs. (Note that a proxy between hand. This includes servers behind NATs. (Note that a proxy between
address domains may be required to get TCP through). address domains may be required to get TCP through).
- Prevents DOS attacks as media receiving client is required to do
STUN responses with authentications on its media reception ports,
see 5.2.6.
Disadvantages: - Improves defenses against DDOS attacks as media receiving client
requires authentications, via STUN on its media reception ports.
See [22] for more details.
Disadvantages:
- Increases the setup delay with at least the amount of time it - Increases the setup delay with at least the amount of time it
takes the server to perform its STUN requests. takes the server to perform its STUN requests.
- Forces servers to use a few well-known media ports.
- Assumes that it is possible to de-multiplex between media packets - Assumes that it is possible to de-multiplex between media packets
and STUN packets. and STUN packets.
- Has high implementation burden for both server and client. Given - Has fairly high implementation burden for both server and client.
the complexity of ICE, it is foreseeable that practitioners may opt Exactly implantation complexity needs to be assessed once ICE is
to use TCP tunneling to deploy RTSP based services. Note that TCP fully defined as a standard. Currently ICE is still a protocol
tunneling can result in loss of real-time properties for the media under development.
streams.
- ICE is not a standard yet. It is only an initial proposal in the
SIPPING working group.
- ICE has the same consideration regarding ALGs as STUN, see section
5.1.5.
Transition:
The use of ICE enables a client to phase-out not needed methods of
creating NAT bindings. However the usage of ICE to ensure that media
delivery is not done to unwanted receiver is not intended to be
removed as it strengthens security.
5.2.6. Security Considerations
ICE has the advantage that it prevents RTSP servers from being used
as DoS tools. The protection is achieved due to the STUN request sent
from the server to the client. A client requesting media gives the
destination address and port for the server to deliver the media too.
The server tries these port using STUN requests. If the client does
not have prior knowledge about the media stream no STUN server are
present. The usage of user name and password ensures that only the
server that the client has requested to deliver media can issue valid
STUN request.
This solution is only vulnerable to a man in the middle attack, where
the attacker can redirect and answer the STUN request before it
reaches the targeted host. If one utilizes a secure channel for the
RTSP messages a potential attacker can't eavesdrop the RTSP messages
carrying the STUN username and password. However an eavesdropper of
the STUN request can still learn them. There exist possibility to
also fend off such attacks, by using HMAC [20] in the STUN request
and send the shared secret in the protected RTSP messages. However
this risk is considered small and the client can also refuse to
answer STUN requests if these requests arrive undesirably frequent,
which may be a sign that someone is trying to break the hash
algorithm in the HMAC code.
The simplest usage scenario of ICE will result in that the RTSP
server utilize a few well known ports for sending media and having
its STUN server available on. The solution does not force this usage
onto the server, as sender ports can be created dynamically at the
time of RTSP DESCRIBE request. However the amount of resources needed
to maintain this usage will be significantly larger then for using a
few well-known ports. The usage of well-known ports will simplify
certain types of attacks on the server, like overload attacks using
STUN.
5.3. Symmetric RTP 6.3. Symmetric RTP
5.3.1. Introduction 6.3.1. Introduction
Symmetric RTP is a NAT traversal solution that is based on requiring Symmetric RTP is a NAT traversal solution that is based on requiring
NATed clients to send UDP packets to the servers media send ports. NATed clients to send UDP packets to the serverÆs media send ports.
In core RTSP, usage of RTP over UDP is uni-directional, where the In core RTSP, usage of RTP over UDP is uni-directional, where the
server sends RTP packets to clients RTP port. Symmetric RTP is server sends RTP packets to clientÆs RTP port. Symmetric RTP is
similar to connection-oriented traffic, where one side (e.g., the similar to connection-oriented traffic, where one side (e.g., the
RTSP client) first "connects" by sending a RTP packet to the other RTSP client) first "connects" by sending a RTP packet to the other
sides RTP port, the recipient then replies to the originating IP and sideÆs RTP port, the recipient then replies to the originating IP and
port. port.
Specifically, when the RTSP server receives the "connect" RTP packet Specifically, when the RTSP server receives the "connect" RTP packet
from its client, it copies the source IP and Port number and uses from its client, it copies the source IP and Port number and uses
them as delivery address for media packets. By having the server send them as delivery address for media packets. By having the server send
media traffic back the same way as the client's packet are sent to media traffic back the same way as the client's packet are sent to
the server, address mappings will be honored. Therefore this the server, address mappings will be honored. Therefore this
technique has the advantage of working for all types of NATs. technique has the advantage of working for all types of NATs.
However, it does require server modifications. Symmetric RTP is However, it does require server modifications. Symmetric RTP is
somewhat more vulnerable to hijacking attacks, which will be somewhat more vulnerable to hijacking attacks, which will be
explained in more details in the section discussing security explained in more details in the section discussing security
concerns. concerns.
5.3.2. Necessary RTSP extensions 6.3.2. 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 level its SETUP request. The RTP SSRC is used to establish some basic level
of security against hijacking attacks. Care must be taken in choosing of security against hijacking attacks. Care must be taken in choosing
clients RTP SSRC. First, it must be unique within all the RTP clientÆs RTP SSRC. First, it must be unique within all the RTP
sessions belonging to the same RTSP session. Secondly, if the RTSP sessions belonging to the same RTSP session. Secondly, if the RTSP
server is sending out media packets to multiple clients from the same server is sending out media packets to multiple clients from the same
send port, the RTP SSRC needs to be unique amongst those clients RTP send port, the RTP SSRC needs to be unique amongst those clientsÆ RTP
sessions. Recognizing that there is a potential that RTP SSRC sessions. Recognizing that there is a potential that RTP SSRC
collision may occur, the RTSP server must be able to signal to client collision may occur, the RTSP server must be able to signal to client
that a collision has occurred and that it wants the client to use a that a collision has occurred and that it wants the client to use a
different RTP SSRC carried in the SETUP response. different RTP SSRC carried in the SETUP response.
A RTP specific "Transport" header parameter is defined to indicate Details of the RTSP extension are beyond the scope of this draft and
that symmetric RTP shall be used for the media transport. The will be defined in a TBD RTSP extension draft.
parameter is included in each "Transport" header specification where
the client will use symmetric RTP. A Server SHALL NOT accept the
transport specification unless it supports symmetric RTP. If the
client has requested to use symmetric RTP for a session the server
MUST include this parameter ("sym_rtp") in the response.
The parameter is defined in ABNF [3] as:
symm-usage = "sym_rtp" "=" "1"
Which follows the definition in [7] for transport parameter
extensions.
It is also necessary to define a "Transport" header parameter,
"client_ssrc", to carry the SSRC that the client will use. In RTP[5],
SSRC parameter is only valid for uni-cast transmission. It identifies
the synchronization source to be associated with the media stream,
and is expressed as an eight-digit hexadecimal value. In cases where
a client will use multiple SSRCs it SHOULD NOT use this parameter.
The parameter is defined in ABNF [3] as:
client_ssrc_def = "client_ssrc" "=" ssrc
Where "ssrc" is defined in [7].
Further, a RTSP options tag that can be used to indicate support of
symmetric RTP according to this specification is defined below:
nat.sym-rtp
This tag SHALL be included in the supported header by both clients
and servers supporting symmetric RTP according to this specification.
5.3.3. Using Symmetric RTP in RTSP
The server and client uses Symmetric RTP in the following way:
1. The client can optionally determine through the use of STUN that
it is located behind a NAT. If the client uses STUN it should
also determine the timeout of NAT it is behind.
2. The client MAY investigate if the server supports symmetric RTP
by including the supported header with the tag "nat.sym-rtp" in
an OPTIONS or DESCRIBE request to the server. A server supporting
symmetric RTP will include the tag in its response.
3. The client determines that it will use symmetric RTP to traverse
the NAT. This decision is based on the knowledge about the NAT
type and the necessary support from the server. If there is no
NAT the client SHOULD NOT use symmetric RTP due to the higher
risk of session hijacking.
4. The client sends a SETUP request which has the parameter
"sym_rtp=1" in the transport line. It MUST also include the
parameter "client_ssrc" in each transport specification
containing "sym_rtp=1". The "client_ssrc" contains the random
SSRC it is going to use for that RTP session, unless in SETUP
response the server over-ride "client_ssrc", in which case the
client must use the server assigned SSRC. The SSRC MUST be
generated in a random way, as the randomness of the SSRC is the
basic security mechanism that prevents hijacking. Symmetric RTP
MUST only be requested for unicast transport.
5. The server chooses the transport specification that it will use
to transport the media. When this transport specification is the
one declaring the use of symmetric RTP the server performs the
following:
- The server MUST include the transport parameters "sym_rtp=1",
and "src_dest" in the response.
- The server MUST both send and receive data on the indicated
ports.
- The server SHALL ignore any of the transport header parameters
"destination", and client_port.
- If the server is using the same UDP send port to send media
packets to multiple RTSP clients, it must also check for client
RTP SSRC collisions. If in a SETUP request, the "client_ssrc"
is already in use, the server must assign a different SSRC that
is unique, and signal it in SETUP response.
6. The Server starts listening on the declared server ports for
RTP/UDP packets containing valid client SSRCs. Any received
RTP/UDP packet not containing a valid client SSRC SHOULD be
ignored. When a RTP/UDP packet containing valid client SSRC is
received, the server looks up the id of the client media session
using the unique client SSRC, stores the source IP and Port as
being the destination address and port for that media session
(i.e., RTP session). It performs the corresponding actions for
the RTCP port to establish the destination of the RTCP
transmissions as well.
7. The client establishes the address binding at the server by
sending RTP or RTCP to the servers declared media address and
port from the port it desires to receive RTP/RTCP on. For the RTP
channel it sends a RTP/UDP packet containing the client SSRC. The
RTP/UDP packet SHALL NOT contain any payload data and use payload
type 0. To the servers RTCP port it sends a normal RTCP packet.
8. Upon reception of a "binding packet" the server SHALL respond. On
the RTP port it SHALL respond with a single RTP/UDP packet using
payload type 0 and having a 0 byte payload. For each received
client packet that contains the correct SSRC the server SHALL
respond with a single packet. For RTCP the client starts
transmitting RTCP packets according to the normal RTCP timing
rules. The server SHALL also send RTCP as soon as it receives a
RTCP packet creating the binding.
9. To ensure that the clients binding packets are not lost the
client SHOULD retransmit the binding RTP packet every 250 ms
until it receives a response with an empty RTP packet or it has
retransmitted 20 times. For RTCP it is enough to transmit RTCP
packet according to the normal rules. However a client MAY send
one RTCP packet every 500 ms until it receives an answer, or it
has tried for 10 seconds.
10. When the client has received answers for both RTP and RTCP it can
safely progress the session and send a PLAY request.
11. To ensure that the binding is not lost the client SHOULD send a
empty RTP/UDP packet with PT=0 to the server every tenth of the
mapping timeout time that has discovered for the NAT. The
discovery can be performed by using STUN. The client SHOULD NOT
send these packets as long as the server transmit RTP packets to
the client. Unless the NAT mappings has very short timeouts or
the RTCP bandwidth is severely restricted the RTCP traffic should
automatically keep its connection open.
5.3.4. Open Issues
The proposal for symmetric RTP contains some open issues that needs
to be addressed.
- Should it be allowed to change a binding once it has been
established? Probably not as it would be security weakness, however
this result in that RTSP SETUP must be used to update the server
destination once a binding has been lost and restored.
- What RTP payload type(s) shall the client use. Should it use one of
the types that the server has declared is going to use in the server
-> client direction or a static one?
- Should the security be improved by having a RTP challenge that can
contain longer random identifiers? If that is the case it should have
a static payload type as the client can't establish dynamic payload
type declarations.
5.3.5. Deployment Considerations 6.3.3. Deployment Considerations
Advantages: Advantages:
- Works for all types of NATs, including those using multiple IP - Works for all types of NATs, including those using multiple IP
addresses. addresses.
- Have no interaction problems with any RTSP ALG changing the - Have no interaction problems with any RTSP ALG changing the
client's information in the transport header. client's information in the transport header.
Disadvantages: Disadvantages:
- Requires Server support. - Requires Server support.
- Has somewhat worse security situation then STUN when using address - Has somewhat worse security situation then STUN when using address
restrictions. restrictions.
- Still requires STUN to discover the timeout of NAT bindings. - Still requires STUN to discover the timeout of NAT bindings.
Transition: 6.3.4. Security Consideration
The usage of symmetric RTP can be phased out as long as the client
has a way of detecting that it does not need it any more. Possible
ways of detecting this is the use of STUN as proposed in the optional
first step. Another way is that it simply is replaced with something
better.
5.3.6. 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.
The method has also no protection if client desires to initiate media The method has also no protection if client desires to initiate media
streams to a target it desires to do a DOS attack on. streams to a target to launch DDOS attacks.
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 with multi-address NATs.
The hijack attack can be performed in various ways. The basic attack The hijack attack can be performed in various ways. The basic attack
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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 SHOULD implement functionality that blocks ports Therefore a server SHOULD implement functionality that blocks ports
that receive multiple binding packets with different invalid SSRCs, that receive multiple binding packets with different invalid SSRCs,
especially when they are coming from the same IP/Port. especially when they are coming from the same IP/Port.
To improve the security against attackers the random tags length To improve the security against attackers the random tags length
could be increased. To achieve a longer random tag while still using could be increased. To achieve a longer random tag while still using
RTP and RTCP, it will be necessary to develop RTP and RTCP payload RTP and RTCP, it will be necessary to develop RTP and RTCP payload
formats for carrying the random tag. formats for carrying the random tag.
5.4. Application Level Gateways 6.4. Application Level Gateways
5.4.1. Introduction 6.4.1. 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
security. As the ALG can't inspect and change the application level security. As the ALG can't inspect and change the application level
messages the protocol will fail due to the middle box. messages the protocol will fail due to the middle box.
2. ALGs need to be updated if extensions to the protocol are added. 2. ALGs need to be updated if extensions to the protocol are added.
Due to deployment issues with changing ALG's this may also break the Due to deployment issues with changing ALG's this may also break the
end-to-end functionality of RTSP. end-to-end functionality of RTSP.
Due to the above reasons it is NOT RECOMMENDED to use an RTSP ALG in Due to the above reasons it is NOT RECOMMENDED to use an RTSP ALG in
NATs. This is especially important for NAT's targeted to home users NATs. This is especially important for NAT's targeted to home users
and small office environments, since it is very hard to upgrade NATs and small office environments, since it is very hard to upgrade NATÆs
deployed in home or SOHO (small office/home office) environment. deployed in home or SOHO (small office/home office) environment.
5.4.2. Guidelines On Writing ALGs for RTSP 6.4.2. Guidelines On Writing ALGs for RTSP
In this section, we provide a step-by-step guideline on how one In this section, we provide a step-by-step guideline on how one
should go about writing an ALG to enable RTSP to traverse a NAT. should go about writing an ALG to enable RTSP to traverse a NAT.
1. Detect any SETUP request. 1. Detect any SETUP request.
2. Try to detect the usage of any of the NAT traversal methods that 2. Try to detect the usage of any of the NAT traversal methods that
replace the address and port of the Transport header parameters replace the address and port of the Transport header parameters
"destination" or "dest_addr". If any of these methods are used, "destination" or "dest_addr". If any of these methods are used,
the ALG SHOULD NOT change the address. Ways to detect that these the ALG SHOULD NOT change the address. Ways to detect that these
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traversal. Otherwise go to next step. traversal. Otherwise go to next step.
6. The ALG SHOULD keep alive the UDP mappings belonging to the an 6. The ALG SHOULD keep alive the UDP mappings belonging to the an
RTSP session as long as: RTSP messages with the session's ID has RTSP session as long as: RTSP messages with the session's ID has
been sent in the last timeout interval, or UDP messages are sent been sent in the last timeout interval, or UDP messages are sent
on any of the UDP mappings during the last timeout interval. on any of the UDP mappings during the last timeout interval.
7. The ALG MAY remove a mapping as soon a TEARDOWN response has been 7. The ALG MAY remove a mapping as soon a TEARDOWN response has been
received for that media stream. received for that media stream.
5.4.3. Deployment Considerations 6.4.3. Deployment Considerations
Advantage: Advantage:
- No impact on either client or server - No impact on either client or server
- Can work for any type of NATs - Can work for any type of NATs
Disadvantage: Disadvantage:
- When deployed they are hard to update to reflect protocol - When deployed they are hard to update to reflect protocol
modifications and extensions. If not updated they will break the modifications and extensions. If not updated they will break the
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- When end-to-end security is used the ALG functionality will fail. - When end-to-end security is used the ALG functionality will fail.
- Can interfere with other type of traversal mechanisms, such as - Can interfere with other type of traversal mechanisms, such as
STUN. STUN.
Transition: Transition:
An RTSP ALG will not be phased out in any automatically way. It must An RTSP ALG will not be phased out in any automatically way. It must
be removed, probably through the removal of the NAT it is associated be removed, probably through the removal of the NAT it is associated
with. with.
5.4.4. Security Considerations 6.4.4. Security Considerations
An ALG will not work when deployment of end-to-end RTSP signaling An ALG will not work when deployment of end-to-end RTSP signaling
security. Therefore deployment of ALG will result in that end-to-end security. Therefore deployment of ALG will result in that end-to-end
security will not be used by clients located behind NATs. security will not be used by clients located behind NATs.
5.5. TCP Tunneling 6.5. TCP Tunneling
5.5.1. Introduction 6.5.1. Introduction
Using a TCP connection that is established from the client to the Using a TCP connection that is established from the client to the
server ensures that the server can send data to the client. The server ensures that the server can send data to the client. The
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 RTP packets. of framing of the RTP packets.
Using TCP also results in that the client has to accept that real- Using TCP also results in that the client has to accept that real-
time performance may no longer be possible. TCP's problem of ensuring time performance may no longer be possible. TCP's problem of ensuring
timely deliver was the reasons why RTP was developed. Problems that timely deliver was the reasons why RTP was developed. Problems that
arise with TCP are: head-of-line blocking, delay introduced by arise with TCP are: head-of-line blocking, delay introduced by
retransmissions, highly varying congestion control. retransmissions, highly varying congestion control.
5.5.2. Usage of TCP tunneling in RTSP 6.5.2. Usage of TCP tunneling in RTSP
The RTSP core specification [7] supports interleaving of media data The RTSP core specification [7] supports interleaving of media data
on the TCP connection that carries RTSP signaling. See section 10.13 on the TCP connection that carries RTSP signaling. See section 10.13
in [7] for how to perform this type of TCP tunneling. in [7] for how to perform this type of TCP tunneling.
There is currently new work on one more way of transporting RTP over There is currently new work on one more way of transporting RTP over
TCP in AVT and MMUSIC. For signaling and rules on how to establish TCP in AVT and MMUSIC. For signaling and rules on how to establish
the TCP connection in lieu of UDP, see [16]. Another draft describes the TCP connection in lieu of UDP, see [16]. Another draft describes
how to frame RTP over the TCP connection is described in [17]. how to frame RTP over the TCP connection is described in [17].
5.5.3. Deployment Considerations 6.5.3. Deployment Considerations
Advantage: Advantage:
- Works through all types of NATs. - Works through all types of NATs where server is in the open.
Disadvantage: Disadvantage:
- Functionality needs to be implemented on both server and client. - Functionality needs to be implemented on both server and client.
- May not give real-time performance. - Will not always meet multimedia streamÆs 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.
5.5.4. Security Considerations 6.5.4. 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 present in RTSP. It is not possible to get any amplification already present in RTSP. It is not possible to get any amplification
effect that is desired for denial of service attacks due to TCP's effect that is desired for denial of service attacks due to TCP's
flow control. flow control.
A possible security consideration would be the performance bottleneck A possible security consideration, when session media data is
when RTSP encryption is applied, since all session media data also interleaved with RTSP, would be the performance bottleneck when RTSP
needs to be encrypted. encryption is applied, since all session media data also needs to be
encrypted.
5.6. TURN (Traversal Using Relay NAT) 6.6. TURN (Traversal Using Relay NAT)
5.6.1. Introduction 6.6.1. Introduction
Traversal Using Relay NAT (TURN) [8] is a protocol for setting up Traversal Using Relay NAT (TURN) [8] is a protocol for setting up
traffic relays that allows clients behind NATs and firewalls to traffic relays that allows 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. before usage.
TURN allows a client to temporarily bind an address/port pair on the TURN allows a client to temporarily bind an address/port pair on the
relay (TURN server) to its local source address/port pair, which is relay (TURN server) to its local source address/port pair, which is
used to contact the TURN server. The TURN server will then forward used to contact the TURN server. The TURN server will then forward
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Using a TURN server makes it possible for a RTSP client to receive Using a TURN server makes it possible for a RTSP client to receive
media streams from even an unmodified RTSP server. However the media streams from even an unmodified RTSP server. However the
problem is that RTSP server may restrict that destinations other than problem is that RTSP server may restrict that destinations other than
the IP address that the RTSP message arrives from shall not be the IP address that the RTSP message arrives from shall not be
accepted. This means that TURN could only be used if the server knows accepted. This means that TURN could only be used if the server knows
and accepts that the IP belongs to a TURN server and the TURN server and accepts that the IP belongs to a TURN server and the TURN server
can't be targeted at an unknown address. Unfortunately TURN servers can't be targeted at an unknown address. Unfortunately TURN servers
can be targeted at any host that has a public IP address by spoofing can be targeted at any host that has a public IP address by spoofing
the source IP of TURN Allocation requests. the source IP of TURN Allocation requests.
5.6.2. Usage of TURN with RTSP 6.6.2. 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.
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
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7. If the client notices that some other source has caused lock down 7. If the client notices that some other source has caused lock down
on the TURN server, the client should create new bindings and on the TURN server, the client should create new bindings and
change the session transport parameters to reflect the new change the session transport parameters to reflect the new
bindings. bindings.
8. If the client pauses and media are not sent for about 75% of the 8. If the client pauses and media are not sent for about 75% of the
mapping timeout the client should use TURN to refresh the mapping timeout the client should use TURN to refresh the
bindings. bindings.
5.6.3. Deployment Considerations 6.6.3. Deployment Considerations
Advantages: Advantages:
- Does not require any server modifications. - Does not require any server modifications.
- Works for any types of NAT as long as the server has public - Works for any types of NAT as long as the server has public
reachable IP address. reachable IP address.
Disadvantage Disadvantage
- TURN is not yet a standard. - TURN is not yet a standard.
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address is valid to be used by the client. address is valid to be used by the client.
- An RTSP ALG MAY change the necessary destinations parameter. This - 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
[8]. However the usage of TURN could be reduced when the demand for [8]. However the usage of TURN could be reduced when the demand for
having NAT traversal is reduced. having NAT traversal is reduced.
5.6.4. Security Considerations 6.6.4. 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 attackers message can present in the RTSP SETUP messages. If the attackerÆs 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
they arrive. This can be accomplished with little risk for the they arrive. This can be accomplished with little risk for the
attacker of being caught, as it can be performed with a spoofed attacker of being caught, as it can be performed with a spoofed
source IP. The client may detect this attack when it receives the source IP. The client may detect this attack when it receives the
lock down packet sent by the attacker as being mal-formatted and not lock down packet sent by the attacker as being mal-formatted and not
corresponding to the expected context. It will also notice the lack corresponding to the expected context. It will also notice the lack
of incoming packets. See bullet 7 in section 5.6.2. of incoming packets. See bullet 7 in section 6.6.2.
The TURN server can also become part of a denial of service attack The TURN server can also become part of a denial of service attack
towards any victim. To perform this attack the attacker must be able towards any victim. To perform this attack the attacker must be able
to eavesdrop on the packets from the TURN server towards a target for to eavesdrop on the packets from the TURN server towards a target for
the DOS attack. The attacker uses the TURN server to setup a RTSP 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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The first attack can be made very hard by applying transport security The first attack can be made very hard by applying transport security
for the RTSP messages, which will hide the TURN servers address and for the RTSP messages, which will hide the TURN servers address and
port numbers from any eavesdropper. port numbers from any eavesdropper.
The second attack requires that the attacker have access to a user The second attack requires that the attacker have access to a user
account on the TURN server to be able set up the TURN mappings. To account on the TURN server to be able set up the TURN mappings. To
prevent this attack the server shall verify that the target prevent this attack the server shall verify that the target
destination accept this media stream. destination accept this media stream.
6. Firewalls 7. Firewalls
Firewalls exist for the purpose of protecting a network from traffic Firewalls exist for the purpose of protecting a network from traffic
not desired by the firewall owner. Therefore it is a policy decision not desired by the firewall owner. Therefore it is a policy decision
if a firewall will let RTSP and its media streams through or not. if a firewall will let RTSP and its media streams through or not.
RTSP is designed to be firewall friendly in that it should be easy to RTSP is designed to be firewall friendly in that it should be easy to
design firewall policies to permit passage of RTSP traffic and its design firewall policies to permit passage of RTSP traffic and its
media streams. media streams.
The firewall will need to allow the media streams associated with a The firewall will need to allow the media streams associated with a
RTSP session pass through it. Therefore the firewall will need an ALG RTSP session pass through it. Therefore the firewall will need an ALG
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open UDP ports for RTP/RTCP. By looking at the source and destination open UDP ports for RTP/RTCP. By looking at the source and destination
addresses and ports the opening in the firewall can be minimized to addresses and ports the opening in the firewall can be minimized to
the least necessary. The opening in the firewall can be closed after the least necessary. The opening in the firewall can be closed after
a teardown message for that session or the session itself times out. a teardown message for that session or the session itself times out.
Simpler firewalls do allow a client to receive media as long as it 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 full cone NAT or a Symmetric NAT. The can have the same behavior as a full cone NAT or a Symmetric NAT. The
only difference is that no address translation is done. To be able to only difference is that no address translation is done. To be able to
use such a firewall a client would need to implement one of the above use such a firewall a client would need to implement one of the above
described NAT traversal methods that includes sending packets to the described NAT traversal methods that include sending packets to the
server to open up the mappings. server to open up the mappings.
7. Open Issues 8. Open Issues
The below list the current open issues with this draft: Some open issues with this draft:
- The lost mappings text needs better text. - At some point we need to recommend one RTSP NAT solution so as to
- Their is need to decide on one of the server modifying schemes and ensure implementations can inter-operate. This decision will
ensure that a stable specification of that method exist. This require that requirements, security and desired goals are evaluated
decision process will require that requirements, security and against implementation cost and the probability to get the final
desired goals are evaluated against implementation cost and solution deployed.
probability to get it deployed. - The ALG recommendations need to be improved and clarified.
- The ALG recommendations needs to be improved and clearer.
- The firewall RTSP ALG recommendations need to be written as they - The firewall RTSP ALG recommendations need to be written as they
are different from the NAT ALG in some perspectives. are different from the NAT ALG in some perspectives.
8. Security Consideration 9. Security Consideration
In preceding sessions we have discussed security merits of each and In preceding sessions we have discussed security merits of each and
every NAT/FW traversal methods for RTSP. In summary, the presence of every NAT/FW traversal methods for RTSP. In summary, the presence of
NAT(s) is a security risk, as a client cannot perform source NAT(s) is a security risk, as a client cannot perform source
authentication of its IP address. This prevents the deployment of any authentication of its IP address. This prevents the deployment of any
future RTSP extensions providing security against hijacking of future RTSP extensions providing security against hijacking of
sessions by a man-in-the-middle. sessions by a man-in-the-middle.
Each of these has security implications. Each of these has security implications.
Using STUN will provide the same level of security as RTSP with out Using STUN will provide the same level of security as RTSP with out
transport level security and source authentications, as long as the transport level security and source authentications; as long as the
server do not grant a client request to send media to different IP server does not grant a client request to send media to different IP
addresses. addresses.
Using symmetric RTP will have a slightly higher risk of session Using symmetric RTP will have a slightly higher risk of session
hijacking than normal RTSP. The reason is that there exists a hijacking than normal RTSP. The reason is that there exists a
probability that an attacker is able to guess the random tag that the probability that an attacker is able to guess the random tag that the
client uses to prove its identity when creating the address bindings. client uses to prove its identity when creating the address bindings.
The usage of an RTSP ALG does not increase in itself the risk for The usage of an RTSP ALG does not increase in itself the risk for
session hijacking. However the deployment of ALGs as sole mechanism session hijacking. However the deployment of ALGs as sole mechanism
for RTSP NAT traversal will prevent deployment of encrypted end-to- for RTSP NAT traversal will prevent deployment of encrypted end-to-
skipping to change at page 33, line 27 skipping to change at page 26, line 17
The usage of TCP tunneling has no known security problems. However it The usage of TCP tunneling has no known security problems. However it
might provide a bottleneck when it comes to end-to-end RTSP signaling might provide a bottleneck when it comes to end-to-end RTSP signaling
security if TCP tunneling is used on a interleaved RTSP signaling security if TCP tunneling is used on a interleaved RTSP signaling
connection. connection.
The usage of TURN has high risk of denial of service attacks against The usage of TURN has high risk of denial of service attacks against
a client. The TURN server can also be used as a redirect point in a 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 DDOS attack unless the server has strict enough rules for who may
create bindings. create bindings.
9. IANA Consideration 10. IANA Consideration
This specification would like to register 2 new Transport header
parameters "sym_rtp" and "client_ssrc" as defined in section 5.3.2.
It does also register one more RTSP feature tag "nat.sym-rtp" as This specification does not define any protocol extensions hence no
defined in section 5.3.2. IANA action is requested.
10. Acknowledgments 11. Acknowledgments
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 specification. group's mailing list that has commented on this specification.
Persons having contributed in such way in no special order to this Persons having contributed in such way in no special order to this
protocol are: Jonathan Rosenberg, Philippe Gentric, Tom Marshall, protocol are: Jonathan Rosenberg, Philippe Gentric, Tom Marshall,
David Yon, Amir Wolf, Anders Klemets, and Colin Perkins. Thomas Zeng David Yon, Amir Wolf, Anders Klemets, and Colin Perkins. Thomas Zeng
would also like to give special thanks to Greg Sherwood of would also like to give special thanks to Greg Sherwood of
PacketVideo for his input into this protocol. PacketVideo for his input into this memo.
11. Author's Addresses 12. Author's Addresses
Magnus Westerlund Tel: +46 8 4048287 Magnus Westerlund Tel: +46 8 4048287
Ericsson Research Email: Magnus.Westerlund@ericsson.com Ericsson Research Email: Magnus.Westerlund@ericsson.com
Ericsson AB Ericsson AB
Torshamnsgatan 23 Torshamnsgatan 23
SE-164 80 Stockholm, SWEDEN SE-164 80 Stockholm, SWEDEN
Thomas Zeng Tel: 1-858-731-5465
PacketVideo Corp. Email: zeng@packetvideo.com Thomas Zeng Tel: 1-858-320-3125
10350 Science Center Dr. PacketVideo
Network Solutions Email: zeng@pvnetsolutions.com
9605 Scranton Rd., Suite 400
San Diego, CA92121 San Diego, CA92121
12. References 13. References
12.1. Normative references 13.1. Normative references
[1] H. Schulzrinne, et. al., "Real Time Streaming Protocol (RTSP)", [1] H. Schulzrinne, et. al., "Real Time Streaming Protocol (RTSP)",
IETF RFC 2326, April 1998. IETF RFC 2326, April 1998.
[2] M. Handley, V. Jacobson, "Session Description Protocol (SDP)", [2] M. Handley, V. Jacobson, "Session Description Protocol (SDP)",
IETF RFC 2327, April 1998. IETF RFC 2327, April 1998.
[3] D. Crocker and P. Overell, "Augmented BNF for syntax specifica- [3] D. Crocker and P. Overell, "Augmented BNF for syntax specifica-
tions: ABNF," RFC 2234, Internet Engineering Task Force, Nov. tions: ABNF," RFC 2234, Internet Engineering Task Force, Nov.
1997. 1997.
[4] S. Bradner, "Key words for use in RFCs to Indicate Requirement [4] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997. Levels", RFC 2119, March 1997.
skipping to change at page 35, line 39 skipping to change at page 27, line 39
Methodology for Network Address Translator (NAT) Traversal for Methodology for Network Address Translator (NAT) Traversal for
the Session Initiation Protocol (SIP)," draft-rosenberg-sipping- the Session Initiation Protocol (SIP)," draft-rosenberg-sipping-
ice-00, IETF draft, February 2003, work in progress. ice-00, IETF draft, February 2003, work in progress.
[10] G. Camarillo, et. al., "Grouping of Media Lines in the Session [10] G. Camarillo, et. al., "Grouping of Media Lines in the Session
Description Protocol (SDP)," IETF RFC 3388, December 2002. Description Protocol (SDP)," IETF RFC 3388, December 2002.
[11] G. Camarillo, J. Rosenberg, " The Alternative Semantics for the [11] G. Camarillo, J. Rosenberg, " The Alternative Semantics for the
Session Description Protocol Grouping Framework," draft- Session Description Protocol Grouping Framework," draft-
camarillo-mmusic-alt-01.txt, IETF draft, June 2002, work in camarillo-mmusic-alt-01.txt, IETF draft, June 2002, work in
progress. progress.
12.2. Informative References 13.2. Informative References
[12] P. Srisuresh, K. Egevang, "Traditional IP Network Address [12] P. Srisuresh, K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)," RFC 3022, Internet Engineering Translator (Traditional NAT)," RFC 3022, Internet Engineering
Task Force, January 2001. Task Force, January 2001.
[13] Tsirtsis, G. and Srisuresh, P., "Network Address Translation - [13] Tsirtsis, G. and Srisuresh, P., "Network Address Translation -
Protocol Translation (NAT-PT)", RFC 2766, Internet Engineering Protocol Translation (NAT-PT)", RFC 2766, Internet Engineering
Task Force, February 2000. Task Force, February 2000.
[14] S. Deering and R. Hinden, "Internet Protocol, Version 6 (IPv6) [14] S. Deering and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, Internet Engineering Task Force, Specification", RFC 2460, Internet Engineering Task Force,
December 1998. December 1998.
skipping to change at page 36, line 21 skipping to change at page 28, line 21
contrans-00.txt, January 2003. contrans-00.txt, January 2003.
[18] D. Daigle, "IAB Considerations for UNilateral Self-Address [18] D. Daigle, "IAB Considerations for UNilateral Self-Address
Fixing (UNSAF) Across Network Address Translation", RFC 3424, Fixing (UNSAF) Across Network Address Translation", RFC 3424,
Internet Engineering Task Force, Nov. 2002 Internet Engineering Task Force, Nov. 2002
[19] R. Finlayason, "IP Multicast and Firewalls", RFC 2588, Internet [19] R. Finlayason, "IP Multicast and Firewalls", RFC 2588, Internet
Engineering Task Force, May 1999 Engineering Task Force, May 1999
[20] Krawczyk, H., Bellare, M., and Canetti, R.: "HMAC: Keyed-hashing [20] Krawczyk, H., Bellare, M., and Canetti, R.: "HMAC: Keyed-hashing
for message authentication". IETF RFC 2104, February 1997 for message authentication". IETF RFC 2104, February 1997
[21] Open Source STUN Server and Client, [21] Open Source STUN Server and Client,
http://www.vovida.org/applications/downloads/stun/index.html http://www.vovida.org/applications/downloads/stun/index.html
[22] Zeng, T.M.: ôMapping ICE (Interactive Connectivity
Establishment) to RTSPö, IETF draft, draft-zeng-mmusic-map-ice-
rtsp-00.txt, Feb 2004
13. IPR Notice 14. IPR Notice
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of claims of rights made available for publication and any assurances of
skipping to change at page 36, line 44 skipping to change at page 29, line 5
obtain a general license or permission for the use of such obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat. be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive this standard. Please address the information to the IETF Executive
Director. Director.
14. Copyright Notice 15. Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and This document and translations of it may be copied and
furnished to others, and derivative works that comment on or furnished to others, and derivative works that comment on or
otherwise explain it or assist in its implementation may be otherwise explain it or assist in its implementation may be
prepared, copied, published and distributed, in whole or in prepared, copied, published and distributed, in whole or in
part, without restriction of any kind, provided that the above part, without restriction of any kind, provided that the above
copyright notice and this paragraph are included on all such copyright notice and this paragraph are included on all such
copies and derivative works. However, this document itself may copies and derivative works. However, this document itself may
not be modified in any way, such as by removing the copyright not be modified in any way, such as by removing the copyright
notice or references to the Internet Society or other Internet notice or references to the Internet Society or other Internet
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assigns. assigns.
This document and the information contained herein is provided This document and the information contained herein is provided
on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE
OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY
IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE. PARTICULAR PURPOSE.
This Internet-Draft expires in December 2003. This Internet-Draft expires in August 2004.
 End of changes. 

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