RFC 7825
A Network Address Translator (NAT) Traversal Mechanism for Media Controlled by the Real-Time Streaming Protocol (RTSP)
Pages: 33
Proposed Standard
Proposed Standard
Part 1 of 2 – Pages 1 to 14
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.
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
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.
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
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.
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
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.
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]>
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
(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.
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.
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 Methods4.5.1. 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"
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.
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.
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