Internet Engineering Task Force (IETF) J. Goldberg

Internet Engineering Task Force (IETF) J. Goldberg
Request for Comments: 7825 Cisco
Category: Standards Track M. Westerlund
ISSN: 2070-1721 Ericsson
T. Zeng
Nextwave Wireless, Inc.
December 2016

A Network Address Translator (NAT) Traversal Mechanism for Media
Controlled by the Real-Time Streaming Protocol (RTSP)

Abstract

This document defines a solution for Network Address Translation
(NAT) traversal for datagram-based media streams set up and
controlled with the Real-Time Streaming Protocol version 2 (RTSP
2.0). It uses Interactive Connectivity Establishment (ICE) adapted
to use RTSP as a signaling channel, defining the necessary RTSP
extensions and procedures.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7825.

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RFC 7825 A Media NAT Traversal Mechanism for RTSP December 2016

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................3
2. Key Words .......................................................4
3. Solution Overview ...............................................4
4. RTSP Extensions .................................................6
4.1. ICE Transport Lower Layer ..................................6
4.2. ICE Candidate Transport Header Parameter ...................8
4.3. ICE Password and Username Transport Header Parameters .....11
4.4. ICE Feature Tag ...........................................11
4.5. Status Codes ..............................................12
4.5.1. 150 Server still working on ICE
connectivity checks ................................12
4.5.2. 480 ICE Connectivity check failure .................12
4.6. New Reason for PLAY_NOTIFY ................................12
4.7. Server-Side SDP Attribute for ICE Support .................13
5. ICE-RTSP .......................................................13
5.1. ICE Features Not Required .................................13
5.1.1. ICE-Lite ...........................................13
5.1.2. ICE-Mismatch .......................................13
5.1.3. ICE Remote Candidate Transport Header Parameter ....14
5.2. High-Reachability Configuration ...........................14
6. Detailed Solution ..............................................14
6.1. Session Description and RTSP DESCRIBE (Optional) ..........14
6.2. Setting Up the Media Streams ..............................15
6.3. RTSP SETUP Request ........................................16
6.4. Gathering Candidates ......................................16
6.5. RTSP Server Response ......................................17
6.6. Server-to-Client ICE Connectivity Checks ..................18
6.7. Client-to-Server ICE Connectivity Check ...................19
6.8. Client Connectivity Checks Complete .......................20
6.9. Server Connectivity Checks Complete .......................20
6.10. Freeing Candidates .......................................20

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6.11. Steady State .............................................21
6.12. Re-SETUP .................................................21
6.13. Server-Side Changes after Steady State ...................22
7. ICE and Proxies ................................................24
7.1. Media-Handling Proxies ....................................24
7.2. Signaling-Only Proxies ....................................25
7.3. Non-supporting Proxies ....................................25
8. RTP and RTCP Multiplexing ......................................26
9. Fallback and Using Partial ICE Functionality to Improve
NAT/Firewall Traversal .........................................27
10. IANA Considerations ...........................................28
10.1. RTSP Feature Tags ........................................28
10.2. Transport Protocol Identifiers ...........................28
10.3. RTSP Transport Parameters ................................29
10.4. RTSP Status Codes ........................................29
10.5. Notify-Reason Value ......................................29
10.6. SDP Attribute ............................................29
11. Security Considerations .......................................30
11.1. ICE and RTSP .............................................30
11.2. Logging ..................................................30
12. References ....................................................31
12.1. Normative References .....................................31
12.2. Informative References ...................................32
Acknowledgments ...................................................33
Authors' Addresses ................................................33

1. Introduction

"Real Time Streaming Protocol (RTSP)" [RFC2326] and RTSP 2.0
[RFC7826] are protocols used to set up and control one or more media
streams delivering media to receivers. It is RTSP's functionality of
setting up media streams that causes serious issues with Network
Address Translators (NATs) [RFC3022] unless extra provisions are made
by the protocol. Thus, there is a need for a NAT traversal mechanism
for the media setup using RTSP.

RTSP 1.0 [RFC2326] has suffered from the lack of a standardized NAT
traversal mechanism for a long time; however, due to quality of the
RTSP 1.0 specification, the work was difficult to specify in an
interoperable fashion. This document is therefore built on the
specification of RTSP 2.0 [RFC7826]. RTSP 2.0 is similar to RTSP 1.0
in many respects, but, significantly for this work, it contains a
well-defined extension mechanism that allows a NAT traversal
extension to be defined that is backwards compatible with RTSP 2.0
peers not supporting the extension. This extension mechanism was not
possible in RTSP 1.0 as it would break RTSP 1.0 syntax and cause
compatibility issues.

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RFC 7825 A Media NAT Traversal Mechanism for RTSP December 2016

There have been a number of suggested ways of resolving the NAT
traversal of media for RTSP, most of which are already used in
implementations. The evaluation of these NAT-traversal solutions in
[RFC7604] has shown that there are many issues to consider. After
extensive evaluation, a mechanism based on Interactive Connectivity
Establishment (ICE) [RFC5245] was selected. There were mainly two
reasons: the mechanism supports RTSP servers behind NATs and the
mechanism mitigates the security threat of using RTSP servers as
Distributed Denial-of-Service (DDoS) attack tools.

This document specifies an ICE-based solution that is optimized for
media delivery from server to client. If future extensions are
specified for other delivery modes than "PLAY", then the
optimizations in regard to when PLAY requests are sent needs to be
reconsidered.

The NAT problem for RTSP signaling traffic is a less prevalent
problem than the NAT problem for RTSP media streams. Consequently,
the former is left for future study.

The ICE usage defined in this specification is called "ICE-RTSP" and
does not match the full ICE for SIP/SDP (Session Description
Protocol) or ICE-Lite as defined in the ICE specification [RFC5245].
ICE-RTSP is tailored to the needs of RTSP and is slightly simpler
than ICE-Full for both clients and servers.

2. Key Words

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].

3. Solution Overview

This overview assumes that the reader has some familiarity with how
ICE [RFC5245] in the context of "SIP: Session Initiation Protocol"
[RFC3261] and "An Offer/Answer Model with the Session Description
Protocol (SDP)" [RFC3264] works, as it primarily points out how the
different ICE steps are accomplished in RTSP.

1. The RTSP server should indicate it has support for ICE via a new
SDP [RFC4566] attribute ("a=rtsp-ice-d-m") in, for example, the
SDP returned in the RTSP DESCRIBE message. This allows RTSP
clients to only perform the new ICE exchanges with servers that
support ICE. If RTSP DESCRIBE is used, the normal capability
determination mechanism should also be used, i.e., Supported

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header with a new ICE feature tag. Note: both mechanisms should
be supported, as there are various use cases where only one of
them is used.

2. The RTSP client reviews the session description returned, for
example by an RTSP DESCRIBE message, to determine what media
streams need to be set up. For each of these media streams
where the transport protocol supports connectivity checks based
on Session Traversal Utilities for (NAT) (STUN) [RFC5389], the
client gathers candidate addresses. See Section 4.1.1 in ICE
[RFC5245]. The client then runs a STUN server on each of the
local candidate's transport addresses it has gathered.

3. The RTSP client sends SETUP requests containing a transport
specification with a lower layer indicating ICE and a new RTSP
Transport header parameter "candidates" listing the ICE
candidates for each media stream.

4. After receiving the list of candidates from a client, the RTSP
server gathers its own candidates. If the server is not behind
a NAT, then a single candidate per address family (e.g., IPv4
and IPv6), media stream, and media component tuple can be
included to reduce the number of combinations and speed up the
completion.

5. The server sets up the media and, if successful, responds to the
SETUP request with a 200 OK response. In that response, the
server selects the transport specification using ICE and
includes its candidates in the candidates parameter.

6. The server starts the connectivity checks following the
procedures described in Sections 5.7 and 5.8 of ICE [RFC5245].
If the server is not behind a NAT and uses a public IP address
with a single candidate per (media stream, component, address
family) tuple, then the server may be configured to not initiate
connectivity checks.

7. The client receives the SETUP response and learns the candidate
addresses to use for the connectivity checks and then initiates
its connectivity check, following the procedures in Section 6 of
ICE [RFC5245].

8. When a connectivity check from the client reaches the server, it
will result in a triggered check from the server. This is why
servers not behind a NAT can wait until this triggered check to
send out any checks for itself, so saving resources and
mitigating the DDoS potential from server-initiated connectivity
checks.

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9. When the client has concluded its connectivity checks, including
nominating candidates, and has correspondingly received the
server connectivity checks on the nominated candidates for all
mandatory components of all media streams, it can issue a PLAY
request. If the connectivity checks have not concluded
successfully, then the client may send a new SETUP request if it
has any new information or believes the server may be able to do
more that can result in successful checks.

10. When the RTSP server receives a PLAY request, it checks to see
that the connectivity checks have concluded successfully, and
only then can it play the stream. If there is a problem with
the checks, then the server sends either a 150 (Server still
working on ICE connectivity checks) response to show that it is
still working on the connectivity checks, or a 480 (ICE
Connectivity check failure) response to indicate a failure of
the checks. If the checks are successful, then the server sends
a 200 OK response and starts delivering media.

The client and server may release unused candidates when the ICE
processing has concluded, a single candidate per component has been
nominated, and a PLAY response has been received (client) or sent
(server).

The client needs to continue to use STUN as a keep-alive mechanism
for the used candidate pairs to keep their NAT bindings current.
RTSP servers behind NATs will also need to send keep-alive messages
when not sending media. This is important since RTSP media sessions
often contain only media traffic from the server to the client so the
bindings in the NAT need to be refreshed by client-to-server traffic
provided by the STUN keep-alive.

4. RTSP Extensions

This section defines the necessary RTSP extensions for performing ICE
with RTSP. Note that these extensions are based on the SDP
attributes in the ICE specification unless expressly indicated
otherwise.

4.1. ICE Transport Lower Layer

A new lower layer "D-ICE" for transport specifications is defined.
This lower layer is datagram clean except that the protocol used must
be possible to demultiplex from STUN messages (see STUN [RFC5389]).
By "datagram clean" we mean that it has to be capable of describing
the length of the datagram, transport that datagram (as a binary
chunk of data), and provide it at the receiving side as one single
item. This lower layer can be any transport type defined for ICE

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that does provide datagram transport capabilities. UDP-based
transport candidates are defined in ICE [RFC5245] and MUST be
supported. It is OPTIONAL to also support TCP-based candidates as
defined by "TCP Candidates with Interactive Connectivity
Establishment (ICE)" [RFC6544]. The TCP-based candidate fulfills the
requirements on providing datagram transport and can thus be used in
combination with RTP. Additional transport types for candidates may
be defined in the future.

This lower layer uses ICE to determine which of the different
candidates shall be used and then, when the ICE processing has
concluded, uses the selected candidate to transport the datagrams
over this transport.

This lower-layer transport can be combined with all upper-layer media
transport protocols that are possible to demultiplex with STUN and
that use datagrams. This specification defines the following
combinations:

o RTP/AVP/D-ICE

o RTP/AVPF/D-ICE

o RTP/SAVP/D-ICE

o RTP/SAVPF/D-ICE

This list can be extended with more transport specifications after
having performed the evaluation that they are compatible with D-ICE
as lower layer. The registration is required to follow the registry
rules for the Transport Protocol Identifier (see Section 22.13.1 of
[RFC7826]).

The lower-layer "D-ICE" has the following rules for the inclusion of
the RTSP Transport header (Section 18.54 of RTSP 2.0 [RFC7826])
parameters:

unicast: ICE only supports unicast operations; thus, it is REQUIRED
that one include the unicast indicator parameter (see
Section 18.54 in RTSP 2.0 [RFC7826]).

candidates: The "candidates" parameter SHALL be included as it
specifies at least one candidate with which to try to establish a
working transport path.

dest_addr: This parameter MUST NOT be included since "candidates" is
used instead to provide the necessary address information.

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ICE-Password: This parameter SHALL be included (see Section 4.2).

ICE-ufrag: This parameter SHALL be included (see Section 4.2).

4.2. ICE Candidate Transport Header Parameter

This section defines a new RTSP transport parameter for carrying ICE
candidates related to the transport specification they appear within,
which may then be validated with an end-to-end connectivity check
using STUN [RFC5389]. Transport parameters may only occur once in
each transport specification. For transport specifications using
"D-ICE" as lower layer, this parameter MUST be present. The
parameter can contain one or more ICE candidates. In the SETUP
response, there is only a single transport specification; if that
uses the "D-ICE" lower layer, this parameter MUST be present and
include the server-side candidates.

The ABNF [RFC5234] for these transport header parameters are:

trns-parameter = <Defined in Section 20.2.3 of [RFC7826]>
trns-parameter =/ SEMI ice-trn-par
ice-trn-par = "candidates" EQUAL DQUOTE SWS ice-candidate
*(SEMI ice-candidate) SWS DQUOTE
ice-candidate = foundation SP
component-id SP
transport SP
priority SP
connection-address SP
port SP
cand-type
[SP rel-addr]
[SP rel-port]
[SP tcp-type-ext] ; Mandatory if transport = TCP
*(SP extension-att-name SP extension-att-value)

foundation = <See Section 15.1 of [RFC5245]>
component-id = <See Section 15.1 of [RFC5245]>
transport = <See Section 15.1 of [RFC5245]>
priority = <See Section 15.1 of [RFC5245]>
cand-type = <See Section 15.1 of [RFC5245]>
rel-addr = <See Section 15.1 of [RFC5245]>
rel-port = <See Section 15.1 of [RFC5245]>
tcp-type-ext = <See Section 4.5 of [RFC6544]>
extension-att-name = <See Section 15.1 of [RFC5245]>
extension-att-value = <See Section 15.1 of [RFC5245]>
connection-address = <See [RFC4566]>
port = <See [RFC4566]>
EQUAL = <Defined in [RFC7826]>

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DQUOTE = <Defined in [RFC7826]>
SWS = <Defined in [RFC7826]>
SEMI = <Defined in [RFC7826]>
SP = <Defined in [RFC7826]>

<connection-address>: is the unicast IP address of the candidate,
allowing for IPv4 addresses, IPv6 addresses, and Fully Qualified
Domain Names (FQDNs), taken from SDP [RFC4566]. Note, this
context MUST have a unicast address for this parameter, even
though a multicast address would be syntactically valid. The
connection address SHOULD use the same format (explicit IP or
FQDN) as in the dest_addr parameter used in the transport
specification that express any fallback. An IP address is
preferred for simplicity, but both an IP Address and FQDN can be
used. In the FQDN case, when receiving a SETUP request or
response containing an FQDN in an ice-candidate parameter, the
FQDN is looked up in the DNS first using a AAAA record (assuming
the agent supports IPv6), and if no result is found or the agent
only supports IPv4, using an A record. If the DNS query returns
more than one IP address, one is chosen, and then used for the
remainder of ICE processing, which in RTSP is subsequent RTSP
SETUPs for the same RTSP session.

<port>: is the port of the candidate; the syntax is defined by SDP
[RFC4566].

<transport>: indicates the transport protocol for the candidate.
The ICE specification defines UDP. "TCP Candidates with
Interactive Connectivity Establishment (ICE)" [RFC6544] defines
how TCP is used as candidates. Additional extensibility is
provided to allow for future transport protocols to be used with
ICE, such as the Datagram Congestion Control Protocol (DCCP)
[RFC4340].

<foundation>: is an identifier that is equivalent for two
candidates that are of the same type, share the same base IP
address, and come from the same STUN server. It is composed of
one to thirty two <ice-char>. The foundation is used to optimize
ICE performance in the Frozen algorithm (as described in
[RFC5245]).

<component-id>: identifies the specific component of the media
stream for which this is a candidate and is a positive integer
belonging to the range 1-256. It MUST start at 1 and MUST
increment by 1 for each component of a particular media stream.
For media streams based on RTP, candidates for the actual RTP
media MUST have a component ID of 1, and candidates for RTCP MUST
have a component ID of 2 unless RTP and RTCP Multiplexing

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(Section 8) is used, in which case the second component is omitted
and RTP and RTCP are both transported over the first component.
Other types of media streams that require multiple components MUST
develop specifications that define the mapping of components to
component IDs. See Section 14 in [RFC5245] for additional
discussion on extending ICE to new media streams.

<priority>: is a positive integer in the range 1 to (2**31 - 1).

<cand-type>: encodes the type of candidate. The ICE specification
defines the values "host", "srflx", "prflx", and "relay" for host,
server-reflexive, peer-reflexive, and relayed candidates,
respectively. The set of candidate types is extensible for the
future.

<rel-addr> and <rel-port>: convey transport addresses related to the
candidate, useful for diagnostics and other purposes. <rel-addr>
and <rel-port> MUST be present for server-reflexive, peer-
reflexive, and relayed candidates. If a candidate is server- or
peer-reflexive, <rel-addr> and <rel-port> are equal to the base
for that server- or peer-reflexive candidate. If the candidate is
relayed, <rel-addr> and <rel-port> are equal to the mapped address
in the TURN Allocate Response that provided the client with that
relayed candidate (see Appendix B.3 of ICE [RFC5245] for a
discussion of its purpose). If the candidate is a host candidate,
<rel-addr> and <rel-port> MUST be omitted.

<tcp-type-ext>: conveys the candidate's connection type (active,
passive, or simultaneous-open (S-O)) for TCP-based candidates.
This MUST be included for candidates that have <transport> set to
TCP and MUST NOT be included for other transport types, including
UDP.

<extension-att-name> and <extension-att-value>: These are prototypes
for future extensions of the candidate line. The ABNF for these
allows any 8-bit value except NUL, CR, or LF. However, the
extensions will occur within a structured line that uses the
DQUOTE, SEMI, SWS, and SP ABNF constructs as delimiters; thus,
those delimiter characters MUST be escaped if they would occur
within an extension-att-name or extension-att-value. The escape
mechanism that MUST be used is the Percent-Encoding defined in
Section 2.1 of [RFC3986]. This mechanism is selected as it needs
to be supported in an RTSP implementation to deal with URIs
anyway. The byte values (in hex) that MUST be escaped are the
following: 0x09, 0x20, 0x22, 0x25, and 0x3B.

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4.3. ICE Password and Username Transport Header Parameters

The ICE password and username for each agent need to be transported
using RTSP. For that purpose, new Transport header parameters are
defined (see Section 18.54 of [RFC7826].

There MUST be an "ICE-Password" and "ICE-ufrag" parameter for each
media stream. The ICE-ufrag and ICE-Password parameter values MUST
be chosen randomly at the beginning of a session. The ICE-ufrag
value MUST contain at least 24 bits of randomness, and the ICE-
Password value MUST contain at least 128 bits of randomness. This
means that the ICE-ufrag value will be at least 4 characters long,
and the ICE-Password value at least 22 characters long, since the
grammar for these attributes allows for 6 bits of randomness per
character. The values MAY be longer than 4 and 22 characters
respectively, of course, up to 256 characters. The upper limit
allows for buffer sizing in implementations. Its large upper limit
allows for increased amounts of randomness to be added over time.

The ABNF [RFC5234] for these parameters is:

trns-parameter =/ SEMI ice-password-par
trns-parameter =/ SEMI ice-ufrag-par
ice-password-par = "ICE-Password" EQUAL DQUOTE password DQUOTE
ice-ufrag-par = "ICE-ufrag" EQUAL DQUOTE ufrag DQUOTE
password = <Defined in [RFC5245], Section 15.4>
ufrag = <Defined in [RFC5245], Section 15.4>
EQUAL = <Defined in [RFC7826]>
SEMI = <Defined in [RFC7826]>
DQUOTE = <Defined in [RFC7826]>

4.4. ICE Feature Tag

A feature tag is defined for use in the RTSP capabilities mechanism
for ICE support of media transport using datagrams: "setup.ice-d-m".
This feature tag indicates that one supports all the mandatory
functions of this specification. It is applicable to all types of
RTSP agents: clients, servers, and proxies.

The RTSP client SHOULD send the feature tag "setup.ice-d-m" in the
Supported header in all SETUP requests that contain the "D-ICE"
lower-layer transport. Note, this is not a "MUST" as an RTSP client
can always attempt to perform a SETUP using ICE to see if it
functions or fails. However, including the feature tag in the
Supported header ensures that proxies supporting this specification
explicitly indicate such support; see Section 7.

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4.5. Status Codes

For ICE, there are two new RTSP response codes to indicate progress
and errors.

+------+----------------------------------------------+-------------+
| Code | Description | Method |
+------+----------------------------------------------+-------------+
| 150 | Server still working on ICE connectivity | PLAY |
| | checks | |
| | | |
| 480 | ICE Connectivity check failure | PLAY, SETUP |
+------+----------------------------------------------+-------------+

Table 1: New Status Codes and Their Usage with RTSP Methods

4.5.1. 150 Server still working on ICE connectivity checks

The 150 response code indicates that ICE connectivity checks are
still in progress and haven't concluded. This response SHALL be sent
within 200 milliseconds of receiving a PLAY request that currently
can't be fulfilled because ICE connectivity checks are still running.
A client can expect network delays between the server and client
resulting in a response longer than 200 milliseconds. Subsequently,
every 3 seconds after the previous one was sent, a 150 reply SHALL be
sent until the ICE connectivity checks conclude either successfully
or in failure, and a final response for the request can be provided.

4.5.2. 480 ICE Connectivity check failure

The 480 client error response code is used in cases when the request
can't be fulfilled due to a failure in the ICE processing, such as
all the connectivity checks have timed out. This error message can
appear either in response to a SETUP request to indicate that no
candidate pair can be constructed or in response to a PLAY request to
indicate that the server's connectivity checks resulted in failure.

4.6. New Reason for PLAY_NOTIFY

A new value used in the PLAY_NOTIFY methods Notify-Reason header is
defined: "ice-restart". This reason indicates that an ICE restart
needs to happen on the identified resource and session.

Notify-Reas-val =/ "ice-restart"

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4.7. Server-Side SDP Attribute for ICE Support

If the server supports the media NAT traversal for RTSP-controlled
sessions as described in this RFC, then the server SHOULD include the
"a=rtsp-ice-d-m" SDP attribute in any SDP (if used) describing
content served by the server. This is a session-level-only
attribute; see [RFC4566].

The ABNF [RFC5234] for the "rtsp-ice-d-m" attribute is:

rtsp-ice-d-m-attr = "a=" "rtsp-ice-d-m"

5. ICE-RTSP

This section discusses differences between the regular ICE usage
defined in [RFC5245] and ICE-RTSP. The reasons for the differences
relate to the clearer client/server roles that RTSP provides and how
the RTSP session establishment signaling occurs within RTSP compared
to SIP/SDP offer/answer.

5.1. ICE Features Not Required

A number of ICE signaling features are not needed with RTSP and are
discussed below.

5.1.1. ICE-Lite

The ICE-Lite attribute SHALL NOT be used in the context of RTSP. The
ICE specification describes two implementations of ICE: Full and
Lite, where hosts that are not behind a NAT are allowed to implement
only Lite. For RTSP, the Lite implementation is insufficient because
it does not cause the media server to send a connectivity check,
which is used to protect against making the RTSP server a denial-of-
service tool.

5.1.2. ICE-Mismatch

The ice-mismatch parameter indicates that the offer arrived with a
default destination for a media component that didn't have a
corresponding candidate attribute. This is not needed for RTSP as
the ICE-based lower-layer transport specification either is supported
or another alternative transport is used. This is always explicitly
indicated in the SETUP request and response.

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5.1.3. ICE Remote Candidate Transport Header Parameter

The Remote candidate attribute is not needed for RTSP for the
following reasons. Each SETUP request results in an independent ICE
processing chain that either fails or results in nominating a single
candidate pair to use. If a new SETUP request for the same media is
sent, it needs to use a new username fragment and password to avoid
any race conditions or uncertainty about to which round of processing
the STUN requests relate.

5.2. High-Reachability Configuration

ICE-RTSP contains a high-reachability configuration when the RTSP
servers are not behind NATs. Please note that "not behind NATs" may
apply in some special cases also for RTSP servers behind NATs given
that they are in an address space that has reachability for all the
RTSP clients intended to able to reach the server. The high-
reachability configuration is similar to ICE-Lite as it allows for
some reduction in the server's burden. However, due to the need to
still verify that the client is actually present and wants to receive
the media stream, the server must also initiate binding requests and
await binding responses. The reduction for the high-reachability
configuration of ICE-RTSP is that they don't need to initiate their
own checks and instead rely on triggered checks for verification.
This also removes a denial-of-service threat where an RTSP SETUP
request will trigger large amount of STUN connectivity checks towards
provided candidate addresses.

6. Detailed Solution

This section describes, in detail, how the interaction and flow of
ICE works with RTSP messages.

6.1. Session Description and RTSP DESCRIBE (Optional)

The RTSP server is RECOMMENDED to indicate it has support for ICE by
sending the "a=rtsp-ice-d-m" SDP attribute in the response to the
RTSP DESCRIBE message if SDP is used. This allows RTSP clients to
only send the new ICE exchanges with servers that support ICE thereby
limiting the overhead on current non-ICE supporting RTSP servers.
When not using RTSP DESCRIBE, it is still RECOMMENDED to use the SDP
attribute for the session description.

A client can also use the DESCRIBE request to determine explicitly if
both server and any proxies support ICE. The client includes the
Supported header with its supported feature tags, including
"setup.ice-d-m". Upon seeing the Supported header, any proxy will
include the Proxy-Supported header with the feature tags it supports.

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The server will echo back the Proxy-Supported header and its own
version of the Supported header so enabling a client to determine
whether or not all involved parties support ICE. Note that even if a
proxy is present in the chain that doesn't indicate support for ICE,
it may still work (see Section 7).

For example:

C->S: DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/2.0
CSeq: 312
User-Agent: PhonyClient 1.2
Accept: application/sdp, application/example
Supported: setup.ice-d-m, setup.rtp.rtcp.mux

S->C: RTSP/2.0 200 OK
CSeq: 312
Date: 23 Jan 1997 15:35:06 GMT
Server: PhonyServer 1.1
Content-Type: application/sdp
Content-Length: 367
Supported: setup.ice-d-m, setup.rtp.rtcp.mux

v=0
o=mhandley 2890844526 2890842807 IN IP4 192.0.2.46
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/lectures/sdp.ps
[email protected] (Seminar Management)
t=2873397496 2873404696
a=recvonly
a=rtsp-ice-d-m
a=control: *
m=audio 3456 RTP/AVP 0
a=control: /audio
m=video 2232 RTP/AVP 31
a=control: /video

6.2. Setting Up the Media Streams

The RTSP client reviews the session description returned, for
example, by an RTSP DESCRIBE message, to determine what media
resources need to be set up. For each of these media streams where
the transport protocol supports ICE connectivity checks, the client
SHALL gather candidate addresses for UDP transport as described in
Section 4.1.1 in ICE [RFC5245] according to standard ICE rather than
the ICE-Lite implementation and according to Section 5 of ICE TCP
[RFC6544] for TCP-based candidates.

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6.3. RTSP SETUP Request

The RTSP client will then send at least one SETUP request per media
stream to establish the media streams required for the desired
session. For each media stream where it desires to use ICE, it MUST
include a transport specification with "D-ICE" as the lower layer,
and each media stream SHALL have its own unique combination of ICE
candidates and ICE-ufrag. This transport specification SHOULD be
placed first in the list to give it highest priority. It is
RECOMMENDED that additional transport specifications be provided as a
fallback in case of proxies that do not support ICE. The RTSP client
will be initiating and thus the controlling party in the ICE
processing. For example (note that some lines are broken in
contradiction with the defined syntax due to space restrictions in
the documenting format):

C->S: SETUP rtsp://server.example.com/fizzle/foo/audio RTSP/2.0
CSeq: 313
Transport: RTP/AVP/D-ICE; unicast; ICE-ufrag=8hhY;
ICE-Password=asd88fgpdd777uzjYhagZg; candidates="
1 1 UDP 2130706431 10.0.1.17 8998 typ host;
2 1 UDP 1694498815 192.0.2.3 45664 typ srflx
raddr 10.0.1.17 rport 8998"; RTCP-mux,
RTP/AVP/UDP; unicast; dest_addr=":6970"/":6971",
RTP/AVP/TCP; unicast;interleaved=0-1
Accept-Ranges: NPT, UTC
User-Agent: PhonyClient/1.2
Supported: setup.ice-d-m, setup.rtp.rtcp.mux

6.4. Gathering Candidates

Upon receiving a SETUP request, the server can determine what media
resource should be delivered and which transport alternatives the
client supports. If one based on D-ICE is on the list of supported
transports and preferred among the supported, the below applies.

The transport specification will indicate which media protocol is to
be used and, based on this and the client's candidates, the server
determines the protocol and if it supports ICE with that protocol.
The server SHALL then gather its UDP candidates according to
Section 4.1.1 in ICE [RFC5245] and any TCP-based ones according to
Section 5 of ICE TCP [RFC6544].

Servers that have an address that is generally reachable by any
client within the address scope the server intends to serve MAY be
specially configured (high-reachability configuration). This special
configuration has the goal of reducing the server-side candidate to
preferably a single one per (address family, media stream, media

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component) tuple. Instead of gathering all possible addresses
including relayed and server-reflexive addresses, the server uses a
single address per address family that the server knows should be
reachable by a client behind one or more NATs. The reason for this
special configuration is twofold: Firstly, it reduces the load on the
server in address gathering and in ICE processing during the
connectivity checks. Secondly, it will reduce the number of
permutations for candidate pairs significantly thus potentially
speeding up the conclusion of the ICE processing. However, note that
using this option on a server that doesn't fulfill the requirement of
being reachable is counterproductive, and it is important that this
is correctly configured.

The above general consideration for servers applies also for TCP-
based candidates. A general implementation should support several
candidate collection techniques and connection types. For TCP-based
candidates, a high-reachability configured server is recommended to
only offer Host candidates. In addition to passive connection types,
the server can select to provide active or S-O connection types to
match the client's candidates.

6.5. RTSP Server Response

The server determines if the SETUP request is successful and, if so,
returns a 200 OK response; otherwise, it returns an error code. At
that point, the server, having selected a transport specification
using the "D-ICE" lower layer, will need to include that transport
specification in the response message. The transport specification
SHALL include the candidates gathered in Section 6.4 in the
"candidates" transport header parameter as well as the server's ICE
username fragment and password. In the case that there are no valid
candidate pairs with the combination of the client and server
candidates, a 480 (ICE Connectivity check failure) error response
SHALL be returned, which MUST include the server's candidates. The
return of a 480 error may allow both the server and client to release
their candidates; see Section 6.10.

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Below is an example of a successful response to the request in
Section 6.3.

S->C: RTSP/2.0 200 OK
CSeq: 313
Session: 12345678
Transport: RTP/AVP/D-ICE; unicast; RTCP-mux; ICE-ufrag=MkQ3;
ICE-Password=pos12Dgp9FcAjpq82ppaF; candidates="
1 1 UDP 2130706431 192.0.2.56 50234 typ host"
Accept-Ranges: NPT
Date: 23 Jan 1997 15:35:06 GMT
Server: PhonyServer 1.1
Supported: setup.ice-d-m, setup.rtp.rtcp.mux

6.6. Server-to-Client ICE Connectivity Checks

The server SHALL start the connectivity checks following the
procedures described in Sections 5.7 and 5.8 of ICE [RFC5245] unless
it is configured to use the high-reachability option. If it is, then
it MAY suppress its own checks until the server's checks are
triggered by the client's connectivity checks.

Please note that Section 5.8 of ICE [RFC5245] does specify that the
initiation of the checks are paced and new ones are only started
every Ta milliseconds. The motivation for this is documented in
Appendix B.1 of ICE [RFC5245] as for SIP/SDP all media streams within
an offer/answer dialog are running using the same queue. To ensure
the same behavior with RTSP, the server SHALL use a single pacer
queue for all media streams within each RTSP session.

The values for the pacing of STUN and TURN transactions Ta and RTO
can be configured but have the same minimum values defined in the ICE
specification.

When a connectivity check from the client reaches the server, it will
result in a triggered check from the server as specified in
Section 7.2.1.4 of ICE [RFC5245]. This is why servers with a high-
reachability address can wait until this triggered check to send out
any checks for itself, so saving resources and mitigating the DDoS
potential.

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6.7. Client-to-Server ICE Connectivity Check

The client receives the SETUP response and learns the candidate
addresses to use for the connectivity checks. The client SHALL
initiate its connectivity check(s), following the procedures in
Section 6 of ICE [RFC5245]. The pacing of STUN transactions
(Appendix B.1 of [RFC5245]) SHALL be used across all media streams
that are part of the same RTSP session.

Aggressive nomination SHOULD be used with RTSP during initial SETUP
for a resource. This doesn't have all the negative impact that it
has in offer/answer as media playing only starts after issuing a PLAY
request. Thus, the issue with a change of the media path being used
for delivery can be avoided by not issuing a PLAY request while STUN
connectivity checks are still outstanding. Aggressive nomination can
result in multiple candidate pairs having their nominated flag set,
but according to Section 8.1.1.2 of ICE [RFC5245], when the PLAY
request is sent, the media will arrive on the pair with the highest
priority. Note, different media resources may still end up with
different foundations.

The above does not change ICE and its handling of aggressive
nomination. When using aggressive nomination, a higher-priority
candidate pair with an outstanding connectivity check message can
move into the Succeeded state and the candidate pair will have its
Nominated flag set. This results in the higher-priority candidate
pair being used instead of the previous pair, which is also in the
Succeeded state.

To avoid this occurring during actual media transport, the RTSP
client can add additional logic when the ICE processing overall is
completed to indicate if there are still higher-priority connectivity
checks outstanding. If some check is still outstanding, the
implementation can choose to wait until some additional timeout is
triggered or the outstanding checks complete before progressing with
a PLAY request. An alternative is to accept the risk for a path
change during media delivery and start playing immediately.

RTSP clients that want to ensure that each media resource uses the
same path can use regular nomination where both 1) the ICE processing
completion criteria and 2) which media streams are nominated for use
can be controlled. This does not affect the RTSP server, as its role
is the one of being controlled.

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6.8. Client Connectivity Checks Complete

When the client has concluded all of its connectivity checks and has
nominated its desired candidate pair for a particular media stream,
it MAY issue a PLAY request for that stream. Note that due to the
aggressive nomination, there is a risk that any outstanding check may
nominate another pair than what was already nominated. The candidate
pair with the highest priority will be used for the media. If the
client has locally determined that its checks have failed, it may try
providing an extended set of candidates and update the server
candidate list by issuing a new SETUP request for the media stream.

If the client concluded its connectivity checks successfully and
therefore sent a PLAY request but the server cannot conclude
successfully, the server will respond with a 480 (ICE Connectivity
check failure) error response. Upon receiving the 480 (ICE
Connectivity check failure) response, the client may send a new SETUP
request assuming it has any new information that can be included in
the candidate list. If the server is still performing the checks
when receiving the PLAY request, it will respond with a 150 (Server
still working on ICE connectivity checks) response to indicate this.

6.9. Server Connectivity Checks Complete

When the RTSP server receives a PLAY request, it checks to see that
the connectivity checks have concluded successfully and only then
will it play the stream. If the PLAY request is for a particular
media stream, the server only needs to check that the connectivity
checks for that stream completed successfully. If the server has not
concluded its connectivity checks, the server indicates that by
sending the 150 (Server still working on ICE connectivity checks)
(Section 4.5.1). If there is a problem with the checks, then the
server sends a 480 response to indicate a failure of the checks. If
the checks are successful, then the server sends a 200 OK response
and starts delivering media.

6.10. Freeing Candidates

Both server and client MAY free their non-selected candidates as soon
as a 200 OK response has been issued/received for the PLAY request
and no outstanding connectivity checks exist.

Clients and servers MAY free all their gathered candidates after
having received or sent, respectively, a 480 response to a SETUP
request. Clients will likely free their candidates first after
having tried any additional actions that may resolve the issue, e.g.,
verifying the address gathering, or use additional STUN or TURN

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servers. Thus, a server will have to weigh the cost of doing address
gathering versus maintaining the gathered address for some time to
allow any new SETUP request to be issued by the client.

If the 480 response is sent in response to a PLAY request, the server
MUST NOT free its gathered candidates. Instead, it will have to wait
for additional actions from the client or terminate the RTSP session
due to inactivity.

6.11. Steady State

The client and server SHALL use STUN to send keep-alive messages for
the nominated candidate pair(s) following the rules of Section 10 of
ICE [RFC5245]. This is important, as normally RTSP play mode
sessions only contain traffic from the server to the client so the
bindings in the NAT need to be refreshed by the client-to-server
traffic provided by the STUN keep-alive.

6.12. Re-SETUP

A client that decides to change any parameters related to the media
stream setup will send a new SETUP request. In this new SETUP
request, the client MAY include a new different ICE username fragment
and password to use in the ICE processing. The new ICE username and
password SHALL cause the ICE processing to start from the beginning
again, i.e., an ICE restart (Section 9.1.1.1 of [RFC5245]). The
client SHALL in case of ICE restart, gather candidates and include
the candidates in the transport specification for D-ICE.

ICE restarts may be triggered due to changes of client or server
attachment to the network, such as changes to the media streams
destination or source address or port. Most RTSP parameter changes
would not require an ICE restart, but would use existing mechanisms
in RTSP to indicate from what point in the RTP stream they apply.
These include the following: performing a pause prior to the
parameter change and then resume; assuming the server supports using
SETUP during the PLAY state; or using the RTP-Info header
(Section 18.45 of [RFC7826]) to indicate from where in the media
stream the change shall apply.

Even if the server does not normally support SETUP during PLAY state,
it SHALL support SETUP requests in PLAY state for the purpose of
changing only the ICE parameters, which are ICE-Password, ICE-ufrag,
and the content of ICE candidates.

If the RTSP session is in playing state at the time of sending the
SETUP request requiring ICE restart, then the ICE connectivity checks
SHALL use Regular nomination. Any ongoing media delivery continues

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on the previously nominated candidate pairs until the new pairs have
been nominated for the individual media stream. Once the nomination
of the new candidate pair has completed, all unused candidates may be
released. If the ICE processing fails and no new candidate pairs are
nominated for use, then the media stream MAY continue to use the
previously nominated candidate pairs while they still function. If
they appear to fail to transport media packets anymore, then the
client can select between two actions: attempting any actions that
might make ICE work or terminating the RTSP session. Firstly, it can
attempt any actions available that might make ICE work, like trying
another STUN/TURN server or changing the transport parameters. In
that case, the client modifies the RTSP session, and if ICE is still
to be used, the client restarts ICE once more. Secondly, if the
client is unable to modify the transport or ICE parameters, it MUST
NOT restart the ICE processing, and it SHOULD terminate the RTSP
session.

6.13. Server-Side Changes after Steady State

A server may require an ICE restart because of server-side load
balancing or a failure resulting in an IP address and a port number
change. In that case, the server SHALL use the PLAY_NOTIFY method to
inform the client (Section 13.5 [RFC7826]) with a new Notify-Reason
header: ice-restart. The server will identify if the change is for a
single media or for the complete session by including the
corresponding URI in the PLAY_NOTIFY request.

Upon receiving and responding to this PLAY_NOTIFY with an ice-restart
reason, the client SHALL gather new ICE candidates and send SETUP
requests for each media stream part of the session. The server
provides its candidates in the SETUP response the same way as for the
first time ICE processing. Both server and client SHALL provide new
ICE usernames and passwords. The client MAY issue the SETUP request
while the session is in PLAYING state.

If the RTSP session is in PLAYING state when the client issues the
SETUP request, the client SHALL use Regular nomination. If not, the
client will use the same procedures as for when first creating the
session.

Note that for each media stream keep-alive messages on the previous
set of candidate pairs SHOULD continue until new candidate pairs have
been nominated. After having nominated a new set of candidate pairs,
the client may continue to receive media for some additional time.
Even if the server stops delivering media over that candidate pair at
the time of nomination, media may arrive for up to one maximum
segment lifetime as defined in TCP (2 minutes). Unfortunately, if
the RTSP server is divided into a separate controller and media

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stream, a failure may result in continued media delivery for a longer
time than the maximum segment lifetime, thus source filtering is
RECOMMENDED.

For example:

S->C: PLAY_NOTIFY rtsp://example.com/fizzle/foo RTSP/2.0
CSeq: 854
Notify-Reason: ice-restart
Session: uZ3ci0K+Ld
Server: PhonyServer 1.1

C->S: RTSP/2.0 200 OK
CSeq: 854
User-Agent: PhonyClient/1.2

C->S: SETUP rtsp://server.example.com/fizzle/foo/audio RTSP/2.0
CSeq: 314
Session: uZ3ci0K+Ld
Transport: RTP/AVP/D-ICE; unicast; ICE-ufrag=Kl1C;
ICE-Password=H4sICGjBsEcCA3Rlc3RzLX; candidates="
1 1 UDP 2130706431 10.0.1.17 8998 typ host;
2 1 UDP 1694498815 192.0.2.3 51456 typ srflx
raddr 10.0.1.17 rport 9002"; RTCP-mux,
RTP/AVP/UDP; unicast; dest_addr=":6970"/":6971",
RTP/AVP/TCP; unicast;interleaved=0-1
Accept-Ranges: NPT, UTC
Supported: setup.ice-d-m, setup.rtp.rtcp.mux
User-Agent: PhonyClient/1.2

C->S: SETUP rtsp://server.example.com/fizzle/foo/video RTSP/2.0
CSeq: 315
Session: uZ3ci0K+Ld
Transport: RTP/AVP/D-ICE; unicast; ICE-ufrag=hZv9;
ICE-Password=JAhA9myMHETTFNCrPtg+kJ; candidates="
1 1 UDP 2130706431 10.0.1.17 9000 typ host;
2 1 UDP 1694498815 192.0.2.3 51576 typ srflx
raddr 10.0.1.17 rport 9000"; RTCP-mux,
RTP/AVP/UDP; unicast; dest_addr=":6972"/":6973",
RTP/AVP/TCP; unicast;interleaved=0-1
Accept-Ranges: NPT, UTC
Supported: setup.ice-d-m, setup.rtp.rtcp.mux
User-Agent: PhonyClient/1.2

S->C: RTSP/2.0 200 OK
CSeq: 314
Session: uZ3ci0K+Ld

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Transport: RTP/AVP/D-ICE; unicast; RTCP-mux; ICE-ufrag=CbDm;
ICE-Password=OfdXHws9XX0eBr6j2zz9Ak; candidates="
1 1 UDP 2130706431 192.0.2.56 50234 typ host"
Accept-Ranges: NPT
Date: 11 March 2011 13:17:46 GMT
Server: PhonyServer 1.1
Supported: setup.ice-d-m, setup.rtp.rtcp.mux

S->C: RTSP/2.0 200 OK
CSeq: 315
Session: uZ3ci0K+Ld
Transport: RTP/AVP/D-ICE; unicast; RTCP-mux; ICE-ufrag=jigs;
ICE-Password=Dgx6fPj2lsa2WI8b7oJ7+s; candidates="
1 1 UDP 2130706431 192.0.2.56 47233 typ host"
Accept-Ranges: NPT
Date: 11 March 2011 13:17:47 GMT
Server: PhonyServer 1.1
Supported: setup.ice-d-m, setup.rtp.rtcp.mux

7. ICE and Proxies

RTSP allows for proxies that can be of two fundamental types
depending on whether or not they relay and potentially cache the
media. Their differing impact on the RTSP NAT traversal solution,
including backwards compatibility, is explained below.

7.1. Media-Handling Proxies

An RTSP proxy that relays or caches the media stream for a particular
media session can be considered to split the media transport into two
parts: firstly, a media transport between the server and the proxy
according to the proxy's need, and, secondly, delivery from the proxy
to the client. This split means that the NAT traversal solution will
be run on each individual media leg according to need.

It is RECOMMENDED that any media-handling proxy support the media NAT
traversal defined within this specification. This is for two
reasons: firstly, to enable clients to perform NAT traversal for the
media between the proxy and itself and secondly to allow the proxy to
be topology independent to support performing NAT traversal (to the
server) for clients not capable of NAT traversal present in the same
address domain as the proxy.

For a proxy to support the media NAT traversal defined in this
specification, a proxy will need to implement the solution fully and
be able to act as both a controlling and a controlled ICE peer. The
proxy also SHALL include the "setup.ice-d-m" feature tag in any
applicable capability negotiation headers, such as Proxy-Supported.

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7.2. Signaling-Only Proxies

A signaling-only proxy handles only the RTSP signaling and does not
have the media relayed through proxy functions. This type of proxy
is not likely to work unless the media NAT traversal solution is in
place between the client and the server, because the DoS protection
measures, as discussed in Section 21.2.1 of RTSP 2.0 [RFC7826],
usually prevent media delivery to addresses other than from where the
RTSP signaling arrives at the server.

The solution for the signaling-only proxy is that it must forward the
RTSP SETUP requests including any transport specification with the
"D-ICE" lower layer and the related transport parameters. A proxy
supporting this functionality SHALL indicate its capability by always
including the "setup.ice-d-m" feature tag in the Proxy-Supported
header in any SETUP request or response.

7.3. Non-supporting Proxies

A media-handling proxy that doesn't support the ICE media NAT
traversal specified here is assumed to remove the transport
specification and use any of the lower prioritized transport
specifications if provided by the requester. The specification of
such a non-ICE transport enables the negotiation to complete,
although with a less preferred method since a NAT between the proxy
and the client may result in failure of the media path.

A non-media-handling proxy is expected to ignore and simply forward
all unknown transport specifications. However, this can only be
guaranteed for proxies following the RTSP 2.0 specification
[RFC7826].

The usage of the "setup.ice-d-m" feature tag in the Proxy-Require
header is NOT RECOMMENDED because it can have contradictory results.
For a proxy that does not support ICE but is media handling, the
inclusion of the feature tag will result in aborting the setup and
indicating that it isn't supported, which is desirable if providing
other fallbacks or other transport configurations to handle the
situation is wanted. For non-ICE-supporting non-media-handling
proxies, the result will be aborting the setup. However, the setup
might have worked if the feature tag wasn't present in the Proxy-
Require header. This variance in results is the reason we don't
recommend the usage of the Proxy-Require header. Instead, we
recommend the usage of the Supported header to force proxies to
include the feature tags for the intersection of what the proxy chain
supports in the Proxy-Supported header. This will provide a positive
indication when all proxies in the chain between the client and
server support the functionality.

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If a proxy doesn't support the "setup.ice-d-m" feature, but that
proxy is not a media-handling proxy, the ICE-based setup could still
work, since such a proxy may do pass through on any transport
parameters. Unfortunately ,the Proxy-Require and Proxy-Supported
RTSP headers failed to provide that information. The only way of
finding whether or not this is the case is to try perform a SETUP
including a Transport header with transport specifications using ICE.

8. RTP and RTCP Multiplexing

"Multiplexing RTP Data and Control Packets on a Single Port"
[RFC5761] specifies how and when RTP and RTCP can be multiplexed on
the same port. This multiplexing is beneficial when combined with
ICE for RTSP as it makes RTP and RTCP need only a single component
per media stream instead of two, so reducing the load on the
connectivity checks. For details on how to negotiate RTP and RTCP
multiplexing, see Appendix C of RTSP 2.0 [RFC7826].

Multiplexing RTP and RTCP has the benefit that it avoids the need for
handling two components per media stream when RTP is used as the
media transport protocol. This eliminates at least one STUN check
per media stream and will also reduce the time needed to complete the
ICE processing by at least the time it takes to pace out the
additional STUN checks of up to one complete round-trip time for a
single media stream. In addition to the protocol performance
improvements, the server and client-side complexities are reduced as
multiplexing halves the total number of STUN instances and holding
the associated state. Multiplexing will also reduce the combinations
and length of the list of possible candidates.

The implementation of RTP and RTCP multiplexing is additional work
required for this solution. However, when implementing the ICE
solution, a server or client will need to implement a demultiplexer
between the STUN and RTP or RTCP packets below the RTP/RTCP
implementation anyway, so the additional work of one new
demultiplexing point directly connected to the STUN and RTP/RTCP
seems small relative to the benefits provided.

Due to the benefits mentioned above, RTSP servers and clients that
support "D-ICE" lower-layer transport in combination with RTP SHALL
also implement and use RTP and RTCP multiplexing as specified in
Appendix C.1.6.4 of [RFC7826] and [RFC5761].

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9. Fallback and Using Partial ICE Functionality to Improve NAT/Firewall
Traversal

The need for fallback from ICE in RTSP should be less than for SIP
using ICE in SDP offer/answer where a default destination candidate
is very important to enable interworking with non-ICE capable
endpoints. In RTSP, capability determination for ICE can happen
prior to the RTSP SETUP request. This means a client should normally
not need to include fallback alternatives when offering ICE, as the
capability for ICE will already be determined. However, as described
in this section, clients may wish to use part of the ICE
functionality to improve NAT/firewall traversal where the server is
not ICE capable.

Section 4.1.4 of the ICE [RFC5245] specification does recommend that
the default destination, i.e., what is used as fallback if the peer
isn't ICE capable, is a candidate of relayed type to maximize the
likelihood of successful transport of media. This is based on the
peer in SIP using SDP offer/answer is almost as likely as the RTSP
client to be behind a NAT. For RTSP, the deployment of servers is
much more heavily weighted towards deployment with public
reachability. In fact, since publicly reachable servers behind NAT
either need to support ICE or have static configurations that allow
traversal, one can assume that the server will have a public address
or support ICE. Thus, the selection of the default destination
address for RTSP can be differently prioritized.

As an ICE-enabled client behind a NAT needs to be configured with a
STUN server address to be able to gather candidates successfully,
this can be used to derive a server reflexive candidate for the
client's port. How useful this is for a NATed RTSP client as a
default candidate depends on the properties of the NAT. As long as
the NAT uses an address-independent mapping, then using a STUN-
derived reflexive candidate is likely to be successful. However,
this is brittle in several ways, and the main reason why the original
specification of STUN [RFC3489] and direct usage for NAT traversal
was obsoleted. First, if the NAT's behavior is attempted to be
determined using STUN as described in [RFC3489], the determined
behavior might not be representative of the behavior encountered in
another mapping. Secondly, filter state towards the ports used by
the server needs to be established. This requires that the server
actually includes both address and ports in its response to the SETUP
request. Thirdly, messages need to be sent to these ports for keep-
alive at a regular interval. How a server reacts to such unsolicited
traffic is unknown. This brittleness may be accepted in fallback due
to lack of support on the server side.

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RFC 7825 A Media NAT Traversal Mechanism for RTSP December 2016

To maximize the likelihood that an RTSP client is capable of
receiving media, a relay-based address should be chosen as the
default fallback address. However, for RTSP clients lacking a relay
server, such as a TURN server, or where usage of such a server has
significant cost associated with it, the usage of a STUN-derived
server reflexive address as client default has a reasonable
likelihood of functioning and may be used as an alternative.

Fallback addresses need to be provided in their own transport
specification using a specifier that does not include the D-ICE
lower-layer transport. Instead, the selected protocol, e.g., UDP,
needs to be explicitly or implicitly indicated. Secondly, the
selected default candidate needs to be included in the SETUP request.
If this candidate is server reflexive or relayed, the aspect of keep-
alive needs to be ensured.

10. IANA Considerations

Per this document, registrations have been made in a number of
registries, both for RTSP and SDP. For all the below registrations,
the contact person on behalf of the IETF WG MMUSIC is Magnus
Westerlund <[email protected]>.

10.1. RTSP Feature Tags

Per this document, one RTSP 2.0 feature tag has been registered in
the "RTSP 2.0 Feature-tags" registry.

setup.ice-d-m: A feature tag representing the support of the ICE-
based establishment of datagram media transport that is capable of
transport establishment through NAT and firewalls. This feature
tag applies to clients, servers, and proxies and indicates support
of all the mandatory functions of this specification.

10.2. Transport Protocol Identifiers

Per this document, a number of transport protocol combinations have
been registered in the RTSP 2.0 "Transport Protocol Identifiers"
registry:

RTP/AVP/D-ICE: RTP using the AVP profile over an ICE-established
datagram flow.

RTP/AVPF/D-ICE: RTP using the AVPF profile over an ICE-established
datagram flow.

RTP/SAVP/D-ICE: RTP using the SAVP profile over an ICE-established
datagram flow.

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RFC 7825 A Media NAT Traversal Mechanism for RTSP December 2016

RTP/SAVPF/D-ICE: RTP using the SAVPF profile over an ICE-established
datagram flow.

10.3. RTSP Transport Parameters

Per this document, three transport parameters have been registered in
the RTSP 2.0's "Transport Parameters" registry.

candidates: Listing the properties of one or more ICE candidates.
See Section 4.2.

ICE-Password: The ICE password used to authenticate the STUN binding
request in the ICE connectivity checks. See Section 4.3.

ICE-ufrag: The ICE username fragment used to authenticate the STUN
binding requests in the ICE connectivity checks. See Section 4.3.

10.4. RTSP Status Codes

Per this document, two assignments have been made in the "RTSP 2.0
Status Codes" registry. See Section 4.5.

10.5. Notify-Reason Value

Per this document, one assignment has been made in the RTSP 2.0
Notify-Reason header value registry. The defined value is:

ice-restart: This Notify-Reason value allows the server to notify
the client about the need for an ICE restart. See Section 4.6.

10.6. SDP Attribute

One SDP attribute has been registered:

SDP Attribute ("att-field"):

Attribute name: rtsp-ice-d-m
Long form: ICE for RTSP datagram media NAT traversal
Type of attribute: Session-level only
Subject to charset: No
Purpose: RFC 7825, Section 4.7
Values: No values defined
Contact: Magnus Westerlund
Email: [email protected]
Phone: +46 10 714 82 87

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RFC 7825 A Media NAT Traversal Mechanism for RTSP December 2016

11. Security Considerations

ICE [RFC5245] and ICE TCP [RFC6544] provide an extensive discussion
on security considerations that apply here as well.

11.1. ICE and RTSP

A long-standing risk with transmitting a packet stream over UDP is
that the host may not be interested in receiving the stream. On
today's Internet, many hosts are behind NATs or operate host
firewalls that do not respond to unsolicited packets with an ICMP
port unreachable error. Thus, an attacker can construct RTSP SETUP
requests with a victim's IP address and cause a flood of media
packets to be sent to a victim. The addition of ICE, as described in
this document, provides protection from the attack described above.
By performing the ICE connectivity check, the media server receives
confirmation that the RTSP client wants the media. While this
protection could also be implemented by requiring the IP addresses in
the SDP match the IP address of the RTSP signaling packet, such a
mechanism does not protect other hosts with the same IP address (such
as behind the same NAT), and such a mechanism would prohibit
separating the RTSP controller from the media play-out device (e.g.,
an IP-enabled remote control and an IP-enabled television); it also
forces RTSP proxies to relay the media streams through them, even if
they would otherwise be only signaling proxies.

To protect against attacks on ICE based on signaling information,
RTSP signaling SHOULD be protected using TLS to prevent eavesdropping
and modification of information.

The STUN amplification attack described in Section 18.5.2 in ICE
[RFC5245] needs consideration. Servers that are able to run
according to the high-reachability option have good mitigation of
this attack as they only send connectivity checks towards an address
and port pair from which they have received an incoming connectivity
check. This means an attacker requires both the capability to spoof
source addresses and to signal the RTSP server a set of ICE
candidates. Independently, an ICE agent needs to implement the
mitigation to reduce the volume of the amplification attack as
described in the ICE specification.

11.2. Logging

The logging of NAT translations is helpful to analysts, particularly
in enterprises, who need to be able to map sessions when
investigating possible issues where the NAT happens. When using
logging on the public Internet, it is possible that the logs are
large and privacy invasive, so procedures for log flushing and

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RFC 7825 A Media NAT Traversal Mechanism for RTSP December 2016

privacy protection SHALL be in place. Care should be taken in the
protection of these logs and consideration taken to log integrity,
privacy protection, and purging logs (retention policies, etc.).
Also, logging of connection errors and other messages established by
this document can be important.

12. References

12.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.

[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761,
DOI 10.17487/RFC5761, April 2010,
<http://www.rfc-editor.org/info/rfc5761>.

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RFC 7825 A Media NAT Traversal Mechanism for RTSP December 2016

[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
March 2012, <http://www.rfc-editor.org/info/rfc6544>.

[RFC7826] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
and M. Stiemerling, Ed., "Real-Time Streaming Protocol
Version 2.0", RFC 7826, DOI 10.17487/RFC7826, December
2016, <http://www.rfc-editor.org/info/rfc7826>.

12.2. Informative References

[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326,
DOI 10.17487/RFC2326, April 1998,
<http://www.rfc-editor.org/info/rfc2326>.

[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
DOI 10.17487/RFC3022, January 2001,
<http://www.rfc-editor.org/info/rfc3022>.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.

[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
DOI 10.17487/RFC3489, March 2003,
<http://www.rfc-editor.org/info/rfc3489>.

[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<http://www.rfc-editor.org/info/rfc4340>.

Goldberg, et al. Standards Track [Page 32]

RFC 7825 A Media NAT Traversal Mechanism for RTSP December 2016

[RFC7604] Westerlund, M. and T. Zeng, "Comparison of Different NAT
Traversal Techniques for Media Controlled by the Real-Time
Streaming Protocol (RTSP)", RFC 7604,
DOI 10.17487/RFC7604, September 2015,
<http://www.rfc-editor.org/info/rfc7604>.

Acknowledgments

The authors would like to thank: Remi Denis-Courmont for suggesting
the method of integrating ICE in RTSP signaling, Dan Wing for help
with the security section and numerous other issues, Ari Keranen for
review of the document and its ICE details, and Flemming Andreasen
and Alissa Cooper for a thorough review. In addition, Bill Atwood
has provided comments and suggestions for improvements.

Authors' Addresses

Jeff Goldberg
Cisco
32 Hamelacha St.
South Netanya 42504
Israel

Phone: +972 9 8927222
Email: [email protected]

Magnus Westerlund
Ericsson
Farogatan 6
Stockholm SE-164 80
Sweden

Phone: +46 8 719 0000
Email: [email protected]

Thomas Zeng
Nextwave Wireless, Inc.
12670 High Bluff Drive
San Diego, CA 92130
United States of America

Phone: +1 858 480 3100
Email: [email protected]

Goldberg, et al. Standards Track [Page 33]