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RFC 7825 - A Network Address Translator (NAT) Traversal Mechanis

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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)


   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

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

   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",
   "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

   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

   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

   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

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





   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

   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])

   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
                    [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

   <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)

   <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

   <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

   <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

   <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 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"

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"


   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.

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.

   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

              o=mhandley 2890844526 2890842807 IN IP4
              s=SDP Seminar
              i=A Seminar on the session description protocol
              e=seminar@example.com (Seminar Management)
              t=2873397496 2873404696
              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.

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 8998 typ host;
                   2 1 UDP 1694498815 45664 typ srflx
                            raddr 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

   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.

   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 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

   When a connectivity check from the client reaches the server, it will
   result in a triggered check from the server as specified in
   Section 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

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 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.

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

   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 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

   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

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

   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

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

   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 8998 typ host;
                    2 1 UDP 1694498815 51456 typ srflx
                            raddr 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 9000 typ host;
                    2 1 UDP 1694498815 51576 typ srflx
                            raddr 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

         Transport: RTP/AVP/D-ICE; unicast; RTCP-mux; ICE-ufrag=CbDm;
                    ICE-Password=OfdXHws9XX0eBr6j2zz9Ak; candidates="
                    1 1 UDP 2130706431 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 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.

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

   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.

   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].

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

   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.

   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 <magnus.westerlund@ericsson.com>.

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"

   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.

   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: magnus.westerlund@ericsson.com
                            Phone: +46 10 714 82 87

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

   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,

   [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,

   [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,

   [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,

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,

   [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,

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,

   [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,

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,

   [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,

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,

   [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,


   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
   32 Hamelacha St.
   South Netanya  42504

   Phone: +972 9 8927222
   Email: jgoldber@cisco.com

   Magnus Westerlund
   Farogatan 6
   Stockholm  SE-164 80

   Phone: +46 8 719 0000
   Email: magnus.westerlund@ericsson.com

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

   Phone: +1 858 480 3100
   Email: thomas.zeng@gmail.com


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