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rfc:rfc6849

Internet Engineering Task Force (IETF) H. Kaplan, Ed. Request for Comments: 6849 Acme Packet Category: Standards Track K. Hedayat ISSN: 2070-1721 EXFO

                                                              N. Venna
                                                               Saperix
                                                              P. Jones
                                                   Cisco Systems, Inc.
                                                           N. Stratton
                                                       BlinkMind, Inc.
                                                         February 2013
       An Extension to the Session Description Protocol (SDP)
     and Real-time Transport Protocol (RTP) for Media Loopback

Abstract

 The wide deployment of Voice over IP (VoIP), real-time text, and
 Video over IP services has introduced new challenges in managing and
 maintaining real-time voice/text/video quality, reliability, and
 overall performance.  In particular, media delivery is an area that
 needs attention.  One method of meeting these challenges is
 monitoring the media delivery performance by looping media back to
 the transmitter.  This is typically referred to as "active
 monitoring" of services.  Media loopback is especially popular in
 ensuring the quality of transport to the edge of a given VoIP, real-
 time text, or Video over IP service.  Today, in networks that deliver
 real-time media, short of running 'ping' and 'traceroute' to the
 edge, administrators are left without the necessary tools to actively
 monitor, manage, and diagnose quality issues with their service.  The
 extension defined herein adds new Session Description Protocol (SDP)
 media types and attributes that enable establishment of media
 sessions where the media is looped back to the transmitter.  Such
 media sessions will serve as monitoring and troubleshooting tools by
 providing the means for measurement of more advanced VoIP, real-time
 text, and Video over IP performance metrics.

Kaplan, et al. Standards Track [Page 1] RFC 6849 SDP Media Loopback February 2013

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 5741.
 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/rfc6849.

Copyright Notice

 Copyright (c) 2013 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.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Kaplan, et al. Standards Track [Page 2] RFC 6849 SDP Media Loopback February 2013

Table of Contents

 1. Introduction ....................................................3
    1.1. Use Cases Supported ........................................4
 2. Terminology .....................................................6
 3. Overview of Operation ...........................................6
    3.1. SDP Offerer Behavior .......................................6
    3.2. SDP Answerer Behavior ......................................7
 4. New SDP Attributes ..............................................7
    4.1. Loopback-Type Attribute ....................................7
    4.2. Loopback-Role Attributes: loopback-source and
         loopback-mirror ............................................8
 5. Rules for Generating the SDP Offer/Answer .......................9
    5.1. Generating the SDP Offer for Loopback Session ..............9
    5.2. Generating the SDP Answer for Loopback Session ............10
    5.3. Offerer Processing of the SDP Answer ......................12
    5.4. Modifying the Session .....................................12
    5.5. Establishing Sessions between Entities behind NATs ........12
 6. RTP Requirements ...............................................13
 7. Payload Formats for Packet Loopback ............................13
    7.1. Encapsulated Payload Format ...............................14
    7.2. Direct Loopback RTP Payload Format ........................16
 8. SRTP Behavior ..................................................17
 9. RTCP Requirements ..............................................18
 10. Congestion Control ............................................18
 11. Examples ......................................................18
    11.1. Offer for Specific Media Loopback Type ...................19
    11.2. Offer for Choice of Media Loopback Type ..................19
    11.3. Answerer Rejecting Loopback Media ........................20
 12. Security Considerations .......................................21
 13. Implementation Considerations .................................22
 14. IANA Considerations ...........................................22
    14.1. SDP Attributes ...........................................22
    14.2. Media Types ..............................................23
 15. Acknowledgements ..............................................31
 16. References ....................................................31
    16.1. Normative References .....................................31
    16.2. Informative References ...................................32

1. Introduction

 The overall quality, reliability, and performance of VoIP, real-time
 text, and Video over IP services rely on the performance and quality
 of the media path.  In order to assure the quality of the delivered
 media, there is a need to monitor the performance of the media
 transport.  One method of monitoring and managing the overall quality
 of real-time VoIP, real-time text, and Video over IP services is

Kaplan, et al. Standards Track [Page 3] RFC 6849 SDP Media Loopback February 2013

 through monitoring the quality of the media in an active session.
 This type of "active monitoring" of services is a method of
 proactively managing the performance and quality of VoIP-based
 services.
 The goal of active monitoring is to measure the media quality of a
 VoIP, real-time text, or Video over IP session.  A way to achieve
 this goal is to request an endpoint to loop media back to the other
 endpoint and to provide media statistics (e.g., RTP Control Protocol
 (RTCP) [RFC3550] and RTCP Extended Reports (RTCP-XR) [RFC3611]
 information).  Another method involves deployment of special
 endpoints that always loop incoming media back for all sessions.
 Although the latter method has been used and is functional, it does
 not scale to support large networks and introduces new network
 management challenges.  Further, it does not offer the granularity of
 testing a specific endpoint that may be exhibiting problems.
 The extension defined in this document introduces new SDP media types
 and attributes that enable establishment of media sessions where the
 media is looped back to the transmitter.  The SDP offer/answer model
 [RFC3264] is used to establish a loopback connection.  Furthermore,
 this extension provides guidelines on handling RTP [RFC3550], as well
 as usage of RTCP [RFC3550] and RTCP-XR [RFC3611] for reporting media-
 related measurements.

1.1. Use Cases Supported

 As a matter of terminology in this document, packets flow from one
 peer acting as a "loopback source", to the other peer acting as a
 "loopback mirror", which in turn returns packets to the loopback
 source.  In advance of the session, the peers negotiate to determine
 which one acts in which role, using the SDP offer/answer exchange.
 The negotiation also includes details such as the type of loopback to
 be used.
 This specification supports three use cases: "encapsulated packet
 loopback", "direct loopback", and "media loopback".  These are
 distinguished by the treatment of incoming RTP packets at the
 loopback mirror.

1.1.1. Encapsulated Packet Loopback

 In the encapsulated packet loopback case, the entire incoming RTP
 packet is encapsulated as payload within an outer RTP packet that is
 specific to this use case and specified in Section 7.1.  The
 encapsulated packet is returned to the loopback source.  The loopback
 source can generate statistics for one-way path performance up to the
 RTP level for each direction of travel by examining sequence numbers

Kaplan, et al. Standards Track [Page 4] RFC 6849 SDP Media Loopback February 2013

 and timestamps in the encapsulating outer RTP header and the
 encapsulated RTP packet payload.  The loopback source can also play
 back the returned media content for evaluation.
 Because the encapsulating RTP packet header extends the packet size,
 it could encounter difficulties in an environment where the original
 RTP packet size is close to the path Maximum Transmission Unit (MTU)
 size.  The encapsulating payload format therefore offers the
 possibility of RTP-level fragmentation of the returned packets.  The
 use of this facility could affect statistics derived for the return
 path.  In addition, the increased bit rate required in the return
 direction may affect these statistics more directly in a restricted-
 bandwidth situation.

1.1.2. Direct Loopback

 In the direct loopback case, the loopback mirror copies the payload
 of the incoming RTP packet into a new RTP packet, using a payload
 format specific to this use case and specified in Section 7.2.  The
 loopback mirror returns the new packet to the packet source.  There
 is no provision in this case for RTP-level fragmentation.
 This use case has the advantage of keeping the packet size the same
 in both directions.  The packet source can compute only two-way path
 statistics from the direct loopback packet header but can play back
 the returned media content.
 It has been suggested that the loopback source, knowing that the
 incoming packet will never be passed to a decoder, can store a
 timestamp and sequence number inside the payload of the packet it
 sends to the mirror, then extract that information from the returned
 direct loopback packet and compute one-way path statistics as in the
 previous case.  Obviously, playout of returned content is no longer
 possible if this is done.

1.1.3. Media Loopback

 In the media loopback case, the loopback mirror submits the incoming
 packet to a decoder appropriate to the incoming payload type.  The
 packet is taken as close as possible to the analog level, then
 re-encoded according to an outgoing format determined by SDP
 negotiation.  The re-encoded content is returned to the loopback
 source as an RTP packet with payload type corresponding to the
 re-encoding format.
 This usage allows troubleshooting at the codec level.  The capability
 for path statistics is limited to what is available from RTCP
 reports.

Kaplan, et al. Standards Track [Page 5] RFC 6849 SDP Media Loopback February 2013

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].
 SDP: Session Description Protocol, as defined in [RFC4566].  This
      document assumes that the SDP offer/answer model is followed,
      per [RFC3264], but does not assume any specific signaling
      protocol for carrying the SDP.
 The following terms are borrowed from [RFC3264] definitions: offer,
 offerer, answer, answerer, and agent.

3. Overview of Operation

 This document defines two loopback 'types', two 'roles', and two
 encoding formats for loopback.  For any given SDP offerer or answerer
 pair, one side is the source of RTP packets, while the other is the
 mirror looping packets/media back.  Those define the two loopback
 roles.  As the mirror, two 'types' of loopback can be performed:
 packet-level or media-level.  When media-level is used, there is no
 further choice of encoding format -- there is only one format:
 whatever is indicated for normal media, since the "looping" is
 performed at the codec level.  When packet-level looping is
 performed, however, the mirror can either send back RTP in an
 encapsulated format or direct loopback format.  The rest of this
 document describes these loopback types, roles, and encoding formats,
 and the SDP offer/answer rules for indicating them.

3.1. SDP Offerer Behavior

 An SDP offerer compliant to this specification and attempting to
 establish a media session with media loopback will include "loopback"
 media attributes for each individual media description in the offer
 message that it wishes to have looped back.  Note that the offerer
 may choose to only request loopback for some media
 descriptions/streams but not others.  For example, it might wish to
 request loopback for a video stream but not audio, or vice versa.
 The offerer will look for the "loopback" media attributes in the
 media description(s) of the response from the SDP answer for
 confirmation that the request is accepted.

Kaplan, et al. Standards Track [Page 6] RFC 6849 SDP Media Loopback February 2013

3.2. SDP Answerer Behavior

 In order to accept a loopback offer (that is, an offer containing
 "loopback" in the media description), an SDP answerer includes the
 "loopback" media attribute in each media description for which it
 desires loopback.
 An answerer can reject an offered stream (either with loopback-source
 or loopback-mirror) if the loopback-type is not specified, the
 specified loopback-type is not supported, or the endpoint cannot
 honor the offer for any other reason.  The loopback request is
 rejected by setting the stream's media port number to zero in the
 answer as defined in RFC 3264 [RFC3264] or by rejecting the entire
 offer (i.e., by rejecting the session request entirely).
 Note that an answerer that is not compliant to this specification and
 that receives an offer with the "loopback" media attributes would
 ignore the attributes and treat the incoming offer as a normal
 request.  If the offerer does not wish to establish a "normal" RTP
 session, it would need to terminate the session upon receiving such
 an answer.

4. New SDP Attributes

 Three new SDP media-level attributes are defined: one indicates the
 type of loopback, and the other two define the role of the agent.

4.1. Loopback-Type Attribute

 This specification defines a new "loopback" attribute, which
 indicates that the agent wishes to perform loopback, and the type of
 loopback that the agent is able to do.  The loopback-type is a value
 media attribute [RFC4566] with the following syntax:
    a=loopback:<loopback-type>
 Following is the Augmented BNF [RFC5234] for loopback-type:
 attribute              =/ loopback-attr
 ; attribute defined in RFC 4566
 loopback-attr          = "loopback:" SP loopback-type
 loopback-type          = loopback-choice [1*SP loopback-choice]
 loopback-choice        = loopback-type-pkt / loopback-type-media
 loopback-type-pkt      = "rtp-pkt-loopback"
 loopback-type-media    = "rtp-media-loopback"

Kaplan, et al. Standards Track [Page 7] RFC 6849 SDP Media Loopback February 2013

 The loopback-type is used to indicate the type of loopback.  The
 loopback-type values are rtp-pkt-loopback and rtp-media-loopback.
 rtp-pkt-loopback: In this mode, the RTP packets are looped back to
    the sender at a point before the encoder/decoder function in the
    receive direction to a point after the encoder/decoder function in
    the send direction.  This effectively re-encapsulates the RTP
    payload with the RTP/UDP/IP headers appropriate for sending it in
    the reverse direction.  Any type of encoding-related functions,
    such as packet loss concealment, MUST NOT be part of this type of
    loopback path.  In this mode, the RTP packets are looped back with
    a new payload type and format.  Section 7 describes the payload
    formats that are to be used for this type of loopback.  This type
    of loopback applies to the encapsulated and direct loopback use
    cases described in Section 1.1.
 rtp-media-loopback: This loopback is activated as close as possible
    to the analog interface and after the decoder so that the RTP
    packets are subsequently re-encoded prior to transmission back to
    the sender.  This type of loopback applies to the media loopback
    use case described in Section 1.1.3.

4.2. Loopback-Role Attributes: loopback-source and loopback-mirror

 The loopback role defines two property media attributes [RFC4566]
 that are used to indicate the role of the agent generating the SDP
 offer or answer.  The syntax of the two loopback-role media
 attributes is as follows:
    a=loopback-source
 and
    a=loopback-mirror
 Following is the Augmented BNF [RFC5234] for loopback-source and
 loopback-mirror:
 attribute             =/ loopback-source / loopback-mirror
 ; attribute defined in RFC 4566
 loopback-source       = "loopback-source"
 loopback-mirror       = "loopback-mirror"
 loopback-source: This attribute specifies that the entity that
    generated the SDP is the media source and expects the receiver of
    the SDP message to act as a loopback mirror.

Kaplan, et al. Standards Track [Page 8] RFC 6849 SDP Media Loopback February 2013

 loopback-mirror: This attribute specifies that the entity that
    generated the SDP will mirror (echo) all received media back to
    the sender of the RTP stream.  No media is generated locally by
    the looping-back entity for transmission in the mirrored stream.
 The "m=" line in the SDP includes all the payload types that will be
 used during the loopback session.  The complete payload space for the
 session is specified in the "m=" line, and the rtpmap attribute is
 used to map from the payload type number to an encoding name denoting
 the payload format to be used.

5. Rules for Generating the SDP Offer/Answer

5.1. Generating the SDP Offer for Loopback Session

 If an offerer wishes to make a loopback request, it includes both the
 loopback-type and loopback-role attributes in a valid SDP offer:
 Example:   m=audio 41352 RTP/AVP 0 8 100
            a=loopback:rtp-media-loopback
            a=loopback-source
            a=rtpmap:0 pcmu/8000
            a=rtpmap:8 pcma/8000
            a=rtpmap:100 G7221/16000/1
 Since media loopback requires bidirectional RTP, its normal direction
 mode is "sendrecv"; the "sendrecv" direction attribute MAY be encoded
 in SDP or not, as per Section 5.1 of [RFC3264], since it is implied
 by default.  If either the loopback source or mirror wishes to
 disable loopback use during a session, the direction mode attribute
 "inactive" MUST be used as per [RFC3264].  The direction mode
 attributes "recvonly" and "sendonly" are incompatible with the
 loopback mechanism and MUST NOT be indicated when generating an SDP
 offer or answer.  When receiving an SDP offer or answer, if
 "recvonly" or "sendonly" is indicated for loopback, the SDP-receiving
 agent SHOULD treat it as a protocol failure of the loopback
 negotiation and terminate the session through its normal means (e.g.,
 by sending a SIP BYE if SIP is used) or reject the offending media
 stream.
 The offerer may offer more than one loopback-type in the SDP offer.
 The port number and the address in the offer (m/c= lines) indicate
 where the offerer would like to receive the media stream(s).  The
 payload type numbers indicate the value of the payload the offerer
 expects to receive.  However, the answer might indicate a subset of
 payload type numbers from those given in the offer.  In that case,
 the offerer MUST only send the payload types received in the answer,
 per normal SDP offer/answer rules.

Kaplan, et al. Standards Track [Page 9] RFC 6849 SDP Media Loopback February 2013

 If the offer indicates rtp-pkt-loopback support, the offer MUST also
 contain either an encapsulated or direct loopback encoding format
 encoding name, or both, as defined in Sections 7.1 and 7.2 of this
 document.  If the offer only indicates rtp-media-loopback support,
 then neither encapsulated nor direct loopback encoding formats apply
 and they MUST NOT be in the offer.
 If loopback-type is rtp-pkt-loopback, the loopback mirror MUST send,
 and the loopback source MUST receive, the looped-back packets encoded
 in one of the two payload formats (encapsulated RTP or direct
 loopback) as defined in Section 7.
 Example:   m=audio 41352 RTP/AVP 0 8 112
            a=loopback:rtp-pkt-loopback
            a=loopback-source
            a=rtpmap:112 encaprtp/8000
 Example:   m=audio 41352 RTP/AVP 0 8 112
            a=loopback:rtp-pkt-loopback
            a=loopback-source
            a=rtpmap:112 rtploopback/8000

5.2. Generating the SDP Answer for Loopback Session

 As with the offer, an SDP answer for loopback follows SDP
 offer/answer rules for the direction attribute, but directions of
 "sendonly" or "recvonly" do not apply for loopback operation.
 The port number and the address in the answer (m/c= lines) indicate
 where the answerer would like to receive the media stream.  The
 payload type numbers indicate the value of the payload types the
 answerer expects to send and receive.
 An answerer includes both the loopback-role and loopback-type
 attributes in the answer to indicate that it will accept the loopback
 request.  When a stream is offered with the loopback-source
 attribute, the corresponding stream in the response will be
 loopback-mirror and vice versa, provided the answerer is capable of
 supporting the requested loopback-type.
 For example, if the offer contains the loopback-source attribute:
    m=audio 41352 RTP/AVP 0 8
    a=loopback:rtp-media-loopback
    a=loopback-source

Kaplan, et al. Standards Track [Page 10] RFC 6849 SDP Media Loopback February 2013

 The answer that is capable of supporting the offer must contain the
 loopback-mirror attribute:
    m=audio 12345 RTP/AVP 0 8
    a=loopback:rtp-media-loopback
    a=loopback-mirror
 If a stream is offered with multiple loopback-type attributes, the
 answer MUST include only one of the loopback types that are accepted
 by the answerer.  The answerer SHOULD give preference to the first
 loopback-type in the SDP offer.
 For example, if the offer contains:
    m=audio 41352 RTP/AVP 0 8 112
    a=loopback:rtp-media-loopback rtp-pkt-loopback
    a=loopback-source
    a=rtpmap:112 encaprtp/8000
 The answer that is capable of supporting the offer and chooses to
 loopback the media using the rtp-media-loopback type must contain:
    m=audio 12345 RTP/AVP 0 8
    a=loopback:rtp-media-loopback
    a=loopback-mirror
 As specified in Section 7, if the loopback-type is rtp-pkt-loopback,
 either the encapsulated RTP payload format or direct loopback RTP
 payload format MUST be used for looped-back packets.
 For example, if the offer contains:
    m=audio 41352 RTP/AVP 0 8 112 113
    a=loopback:rtp-pkt-loopback
    a=loopback-source
    a=rtpmap:112 encaprtp/8000
    a=rtpmap:113 rtploopback/8000

Kaplan, et al. Standards Track [Page 11] RFC 6849 SDP Media Loopback February 2013

 The answer that is capable of supporting the offer must contain one
 of the following:
    m=audio 12345 RTP/AVP 0 8 112
    a=loopback:rtp-pkt-loopback
    a=loopback-mirror
    a=rtpmap:112 encaprtp/8000
    m=audio 12345 RTP/AVP 0 8 113
    a=loopback:rtp-pkt-loopback
    a=loopback-mirror
    a=rtpmap:113 rtploopback/8000
 The previous examples used the 'encaprtp' and 'rtploopback' encoding
 names, which will be defined in Sections 7.1.3 and 7.2.3.

5.3. Offerer Processing of the SDP Answer

 If the received SDP answer does not contain an a=loopback-mirror or
 a=loopback-source attribute, it is assumed that the loopback
 extensions are not supported by the remote agent.  This is not a
 protocol failure and instead merely completes the SDP offer/answer
 exchange with whatever normal rules apply; the offerer MAY decide to
 end the established RTP session (if any) through normal means of the
 upper-layer signaling protocol (e.g., by sending a SIP BYE).

5.4. Modifying the Session

 At any point during the loopback session, either participant MAY
 issue a new offer to modify the characteristics of the previous
 session, as defined in Section 8 of RFC 3264 [RFC3264].  This also
 includes transitioning from a normal media processing mode to
 loopback mode, and vice versa.

5.5. Establishing Sessions between Entities behind NATs

 Interactive Connectivity Establishment (ICE) [RFC5245], Traversal
 Using Relays around NAT (TURN) [RFC5766], and Session Traversal
 Utilities for NAT (STUN) [RFC5389] provide a general solution to
 establishing media sessions between entities that are behind Network
 Address Translators (NATs).  Loopback sessions that involve one or
 more endpoints behind NATs can also use these general solutions
 wherever possible.
 If ICE is not supported, then in the case of loopback, the mirroring
 entity will not send RTP packets and therefore will not automatically
 create the NAT pinhole in the way that other SIP sessions do.
 Therefore, if the mirroring entity is behind a NAT, it MUST send some

Kaplan, et al. Standards Track [Page 12] RFC 6849 SDP Media Loopback February 2013

 packets to the identified address/port(s) of the peer, in order to
 open the NAT pinhole.  Using ICE, this would be accomplished with the
 STUN connectivity check process or through a TURN server connection.
 If ICE is not supported, either [RFC6263] or Section 10 of ICE
 [RFC5245] can be followed to open the pinhole and keep the NAT
 binding alive/refreshed.
 Note that for any form of NAT traversal to function, symmetric
 RTP/RTCP [RFC4961] MUST be used, unless the mirror can control the
 NAT(s) in its path to create explicit pinholes.  In other words, both
 agents MUST send packets from the source address and port they
 receive packets on, unless some mechanism is used to avoid that need
 (e.g., by using the Port Control Protocol).

6. RTP Requirements

 A loopback source MUST NOT send multiple source streams on the same
 5-tuple, since there is no means for the mirror to indicate which is
 which in its mirrored RTP packets.
 A loopback mirror that is compliant to this specification and accepts
 media with the loopback type rtp-pkt-loopback loops back the incoming
 RTP packets using either the encapsulated RTP payload format or the
 direct loopback RTP payload format as defined in Section 7 of this
 specification.
 A device that is compliant to this specification and performing the
 mirroring using the loopback type rtp-media-loopback MUST transmit
 all received media back to the sender, unless congestion feedback or
 other lower-layer constraints prevent it from doing so.  The incoming
 media is treated as if it were to be played; for example, the media
 stream may receive treatment from Packet Loss Concealment (PLC)
 algorithms.  The mirroring entity re-generates all the RTP header
 fields as it would when transmitting media.  The mirroring entity MAY
 choose to encode the loopback media according to any of the media
 descriptions supported by the offering entity.  Furthermore, in cases
 where the same media type is looped back, the mirroring entity can
 choose to preserve the number of frames/packets and the bit rate of
 the encoded media according to the received media.

7. Payload Formats for Packet Loopback

 The payload formats described in this section MUST be used by a
 loopback mirror when 'rtp-pkt-loopback' is the specified
 loopback-type.  Two different formats are specified here -- an
 encapsulated RTP payload format and a direct loopback RTP payload
 format.  The encapsulated RTP payload format should be used when the
 incoming RTP header information needs to be preserved during the

Kaplan, et al. Standards Track [Page 13] RFC 6849 SDP Media Loopback February 2013

 loopback operation.  This is useful in cases where the loopback
 source needs to measure performance metrics in both directions.
 However, this comes at the expense of increased packet size as
 described in Section 7.1.  The direct loopback RTP payload format
 should be used when bandwidth requirements prevent the use of the
 encapsulated RTP payload format.

7.1. Encapsulated Payload Format

 A received RTP packet is encapsulated in the payload section of the
 RTP packet generated by a loopback mirror.  Each received packet is
 encapsulated in a separate encapsulating RTP packet; the encapsulated
 packet would be fragmented only if required (for example, due to MTU
 limitations).

7.1.1. Usage of RTP Header Fields

 Payload Type (PT): The assignment of an RTP payload type for this
    packet format is outside the scope of this document; it is either
    specified by the RTP profile under which this payload format is
    used or more likely signaled dynamically out-of-band (e.g., using
    SDP; Section 7.1.3 defines the name binding).
 Marker (M) bit: If the received RTP packet is looped back in multiple
    encapsulating RTP packets, the M bit is set to 1 in every fragment
    except the last packet; otherwise, it is set to 0.
 Extension (X) bit: This bit is defined by the RTP profile used.
 Sequence Number: The RTP sequence number SHOULD be generated by the
    loopback mirror in the usual manner with a constant random offset
    as described in RFC 3550 [RFC3550].
 Timestamp: The RTP timestamp denotes the sampling instant for when
    the loopback mirror is transmitting this packet to the loopback
    source.  The RTP timestamp MUST use the same clock rate as that of
    the encapsulated packet.  The initial value of the timestamp
    SHOULD be random for security reasons (see Section 5.1 of RFC 3550
    [RFC3550]).
 Synchronization source (SSRC): This field is set as described in
    RFC 3550 [RFC3550].
 The CSRC count (CC) and contributing source (CSRC) fields are used as
 described in RFC 3550 [RFC3550].

Kaplan, et al. Standards Track [Page 14] RFC 6849 SDP Media Loopback February 2013

7.1.2. RTP Payload Structure

 The outer RTP header of the encapsulating packet is followed by the
 payload header defined in this section, after any header
 extension(s).  If the received RTP packet has to be looped back in
 multiple encapsulating packets due to fragmentation, the
 encapsulating RTP header in each packet is followed by the payload
 header defined in this section.  The header is devised so that the
 loopback source can decode looped-back packets in the presence of
 moderate packet loss [RFC3550].  The RTP payload of the encapsulating
 RTP packet starts with the payload header defined in this section.
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                         receive timestamp                     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | F | R |  CC   |M|     PT      |       sequence number         |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                           transmit timestamp                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |           synchronization source (SSRC) identifier            |
 +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
 |            contributing source (CSRC) identifiers             |
 |                             ....                              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          Figure 1.  Encapsulating RTP Packet Payload Header
 The 12 octets after the receive timestamp are identical to the
 encapsulated RTP header of the received packet except for the first 2
 bits of the first octet.  In effect, the received RTP packet is
 encapsulated by creating a new outer RTP header followed by 4 new
 bytes of a receive timestamp, followed by the original received RTP
 header and payload, except that the first two bits of the received
 RTP header are overwritten as defined here.
 Receive timestamp: 32 bits
 The receive timestamp denotes the sampling instant for when the last
 octet of the received media packet that is being encapsulated by the
 loopback mirror is received from the loopback source.  The same clock
 rate MUST be used by the loopback source.  The initial value of the
 timestamp SHOULD be random for security reasons (see Section 5.1 of
 RFC 3550 [RFC3550]).

Kaplan, et al. Standards Track [Page 15] RFC 6849 SDP Media Loopback February 2013

 Fragmentation (F): 2 bits
 Possible values are First Fragment (00), Last Fragment (01),
 No Fragmentation (10), or Intermediate Fragment (11).  This field
 identifies how much of the received packet is encapsulated in this
 packet by the loopback mirror.  If the received packet is not
 fragmented, this field is set to 10; otherwise, the packet that
 contains the first fragments sets this field to 00.  The packet that
 contains the last fragment sets this field to 01, and all other
 packets set this field to 11.

7.1.3. Usage of SDP

 The payload type number for the encapsulated stream can be negotiated
 using SDP.  There is no static payload type assignment for the
 encapsulating stream, so dynamic payload type numbers MUST be used.
 The binding to the name is indicated by an rtpmap attribute.  The
 name used in this binding is "encaprtp".
 The following is an example SDP fragment for encapsulated RTP.
 m=audio 41352 RTP/AVP 112
 a=rtpmap:112 encaprtp/8000

7.2. Direct Loopback RTP Payload Format

 The direct loopback RTP payload format can be used in scenarios where
 the 16-byte overhead of the encapsulated payload format is of
 concern, or simply due to local policy.  When using this payload
 format, the receiver loops back each received RTP packet payload (not
 header) in a separate RTP packet.
 Because a direct loopback format does not retain the original RTP
 headers, there will be no indication of the original payload-type
 sent to the mirror, in looped-back packets.  Therefore, the loopback
 source SHOULD only send one payload type per loopback RTP session if
 direct mode is used.

7.2.1. Usage of RTP Header Fields

 Payload Type (PT): The assignment of an RTP payload type for the
    encapsulating packet format is outside the scope of this document;
    it is either specified by the RTP profile under which this payload
    format is used or more likely signaled dynamically out-of-band
    (e.g., using SDP; Section 7.2.3 defines the name binding).
 Marker (M) bit: This bit is set to the value in the received packet.

Kaplan, et al. Standards Track [Page 16] RFC 6849 SDP Media Loopback February 2013

 Extension (X) bit: This bit is defined by the RTP profile used.
 Sequence Number: The RTP sequence number SHOULD be generated by the
    loopback mirror in the usual manner with a constant random offset,
    as per [RFC3550].
 Timestamp: The RTP timestamp denotes the sampling instant for when
    the loopback mirror is transmitting this packet to the loopback
    source.  The same clock rate MUST be used as that of the received
    RTP packet.  The initial value of the timestamp SHOULD be random
    for security reasons (see Section 5.1 of RFC 3550 [RFC3550]).
 SSRC: This field is set as described in RFC 3550 [RFC3550].
 The CC and CSRC fields are used as described in RFC 3550 [RFC3550].

7.2.2. RTP Payload Structure

 This payload format does not define any payload-specific headers.
 The loopback mirror simply copies the RTP payload data from the
 payload portion of the RTP packet received from the loopback source.

7.2.3. Usage of SDP

 The payload type number for the payload loopback stream can be
 negotiated using a mechanism like SDP.  There is no static payload
 type assignment for the stream, so dynamic payload type numbers MUST
 be used.  The binding to the name is indicated by an rtpmap
 attribute.  The name used in this binding is "rtploopback".
 The following is an example SDP fragment for the direct loopback RTP
 format.
 m=audio 41352 RTP/AVP 112
 a=rtpmap:112 rtploopback/8000

8. SRTP Behavior

 Secure RTP (SRTP) [RFC3711] MAY be used for loopback sessions.  SRTP
 operates at a lower logical layer than RTP, and thus if both sides
 negotiate to use SRTP, each side uses its own key and performs
 encryption/decryption, authentication, etc.  Therefore, the loopback
 function on the mirror occurs after the SRTP packet has been
 decrypted and authenticated, as a normal cleartext RTP packet without
 a Master Key Identifier (MKI) or authentication tag; once the

Kaplan, et al. Standards Track [Page 17] RFC 6849 SDP Media Loopback February 2013

 cleartext RTP packet or payload is mirrored -- either at the media-
 layer, direct packet-layer, or encapsulated packet-layer -- it is
 encrypted by the mirror using its own key.
 In order to provide the same level of protection to both forward and
 reverse media flows (media to and from the mirror), if SRTP is used
 it MUST be used in both directions with the same properties.

9. RTCP Requirements

 The use of the loopback attribute is intended for the monitoring of
 media quality of the session.  Consequently, the media performance
 information should be exchanged between the offering and the
 answering entities.  An offering or answering agent that is compliant
 to this specification SHOULD support RTCP per [RFC3550] and RTCP-XR
 per RFC 3611 [RFC3611].  Furthermore, if the offerer or answerer
 chooses to support RTCP-XR, they SHOULD support the RTCP-XR Loss Run
 Length Encoding (RLE) Report Block, Duplicate RLE Report Block,
 Statistics Summary Report Block, and VoIP Metrics Report Block per
 Sections 4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611].  The offerer
 and the answerer MAY support other RTCP-XR reporting blocks as
 defined by RFC 3611 [RFC3611].

10. Congestion Control

 All the participants in a media-level loopback session SHOULD
 implement congestion control mechanisms as defined by the RTP profile
 under which the loopback mechanism is implemented.  For audio/video
 profiles, implementations SHOULD conform to the mechanism defined in
 Section 2 of RFC 3551 [RFC3551].
 For packet-level loopback types, the loopback source SHOULD implement
 congestion control.  The mirror will simply reflect back the RTP
 packets it receives (either in encapsulated or direct modes);
 therefore, the source needs to control the congestion of both forward
 and reverse paths by reducing its sending rate to the mirror.  This
 keeps the loopback mirror implementation simpler and provides more
 flexibility for the source performing a loopback test.

11. Examples

 This section provides examples for media descriptions using SDP for
 different scenarios.  The examples are given for SIP-based
 transactions; for convenience, they are abbreviated and do not show
 the complete signaling.

Kaplan, et al. Standards Track [Page 18] RFC 6849 SDP Media Loopback February 2013

11.1. Offer for Specific Media Loopback Type

 An agent sends an SDP offer that looks like:
 v=0
 o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
 s=-
 c=IN IP4 host.atlanta.example.com
 t=0 0
 m=audio 49170 RTP/AVP 0
 a=loopback:rtp-media-loopback
 a=loopback-source
 a=rtpmap:0 pcmu/8000
 The agent is offering to source the media and expects the answering
 agent to mirror the RTP stream per the loopback type
 rtp-media-loopback.
 An answering agent sends an SDP answer that looks like:
 v=0
 o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
 s=-
 c=IN IP4 host.biloxi.example.com
 t=0 0
 m=audio 49270 RTP/AVP 0
 a=loopback:rtp-media-loopback
 a=loopback-mirror
 a=rtpmap:0 pcmu/8000
 The answerer agrees to mirror the media from the offerer at the media
 level.

11.2. Offer for Choice of Media Loopback Type

 An agent sends an SDP offer that looks like:
 v=0
 o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
 s=-
 c=IN IP4 host.atlanta.example.com
 t=0 0
 m=audio 49170 RTP/AVP 0 112 113
 a=loopback:rtp-media-loopback rtp-pkt-loopback
 a=loopback-source
 a=rtpmap:0 pcmu/8000
 a=rtpmap:112 encaprtp/8000
 a=rtpmap:113 rtploopback/8000

Kaplan, et al. Standards Track [Page 19] RFC 6849 SDP Media Loopback February 2013

 The offerer is offering to source the media and expects the answerer
 to mirror the RTP stream at either the media or RTP level.
 An answering agent sends an SDP answer that looks like:
 v=0
 o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
 s=-
 c=IN IP4 host.biloxi.example.com
 t=0 0
 m=audio 49270 RTP/AVP 0 112
 a=loopback:rtp-pkt-loopback
 a=loopback-mirror
 a=rtpmap:0 pcmu/8000
 a=rtpmap:112 encaprtp/8000
 The answerer agrees to mirror the media from the offerer at the
 packet level using the encapsulated RTP payload format.

11.3. Answerer Rejecting Loopback Media

 An agent sends an SDP offer that looks like:
 v=0
 o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
 s=-
 c=IN IP4 host.atlanta.example.com
 t=0 0
 m=audio 49170 RTP/AVP 0
 a=loopback:rtp-media-loopback
 a=loopback-source
 a=rtpmap:0 pcmu/8000
 The offerer is offering to source the media and expects the answerer
 to mirror the RTP stream at the media level.
 An answering agent sends an SDP answer that looks like:
 v=0
 o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
 s=-
 c=IN IP4 host.biloxi.example.com
 t=0 0
 m=audio 0 RTP/AVP 0
 a=rtpmap:0 pcmu/8000

Kaplan, et al. Standards Track [Page 20] RFC 6849 SDP Media Loopback February 2013

 Note in this case that the answerer did not indicate loopback
 support, although it could have and still used a port number of 0 to
 indicate that it does not wish to accept that media session.
 Alternatively, the answering agent could have simply rejected the
 entire SDP offer through some higher-layer signaling protocol means
 (e.g., by rejecting the SIP INVITE request if the SDP offer was in
 the INVITE).

12. Security Considerations

 The security considerations of [RFC3264] and [RFC3550] apply.
 Given that media loopback may be automated without the end user's
 knowledge, the answerer of the media loopback should be aware of
 denial-of-service attacks.  It is RECOMMENDED that session requests
 for media loopback be authenticated and the frequency of such
 sessions limited by the answerer.
 If the higher-layer signaling protocol were not authenticated, a
 malicious attacker could create a session between two parties the
 attacker wishes to target, with each party acting as the loopback
 mirror to the other, of the rtp-pkt-loopback type.  A few RTP packets
 sent to either party would then infinitely loop among the two, as
 fast as they could process them, consuming their resources and
 network bandwidth.
 Furthermore, media loopback provides a means of attack indirection,
 whereby a malicious attacker creates a loopback session as the
 loopback source and uses the mirror to reflect the attacker's packets
 against a target -- perhaps a target the attacker could not reach
 directly, such as one behind a firewall, for example.  Or, the
 attacker could initiate the session as the loopback mirror, in the
 hopes of making the peer generate media against another target.
 If end-user devices such as mobile phones answer loopback requests
 without authentication and without notifying the end user, then an
 attacker could cause the battery to drain, and possibly deny the end
 user normal phone service or cause network data usage fees.  This
 could even occur naturally if a legitimate loopback session does not
 terminate properly and the end device does not have a timeout
 mechanism for such.
 For the reasons noted above, end-user devices SHOULD provide a means
 of indicating to the human user that the device is in a loopback
 session, even if it is an authenticated session.  Devices that answer

Kaplan, et al. Standards Track [Page 21] RFC 6849 SDP Media Loopback February 2013

 or generate loopback sessions SHOULD either perform keepalive/refresh
 tests of the session state through some means or time out the session
 automatically.

13. Implementation Considerations

 The media loopback approach described in this document is a complete
 solution that would work under all scenarios.  However, it is
 possible that the solution may not be lightweight enough for some
 implementations.  In light of this concern, this section clarifies
 which features of the loopback proposal MUST be implemented for all
 implementations and which features MAY be deferred if the complete
 solution is not desired.
 All implementations MUST at least support the rtp-pkt-loopback mode
 for loopback-type, with direct media loopback payload encoding.  In
 addition, for the loopback role, all implementations of an SDP
 offerer MUST at least be able to act as a loopback source.  These
 requirements are intended to provide a minimal level of
 interoperability between different implementations.

14. IANA Considerations

14.1. SDP Attributes

 This document defines three new media-level SDP attributes.  IANA has
 registered the following attributes.
    Contact name:             Kaynam Hedayat
    Email address:            kh274@cornell.edu
    Telephone number:         +1-617-899-3279
    Attribute name:           loopback
    Type of attribute:        Media level.
    Subject to charset:       No.
    Purpose of attribute:     The 'loopback' attribute is used to
                              indicate the type of media loopback.
    Allowed attribute values: The parameters for 'loopback' may be
                              one or more of "rtp-pkt-loopback" and
                              "rtp-media-loopback".  See Section 4
                              of RFC 6849 for syntax.

Kaplan, et al. Standards Track [Page 22] RFC 6849 SDP Media Loopback February 2013

    Contact name:             Kaynam Hedayat
    Email address:            kh274@cornell.edu
    Telephone number:         +1-617-899-3279
    Attribute name:           loopback-source
    Type of attribute:        Media level.
    Subject to charset:       No.
    Purpose of attribute:     The 'loopback-source' attribute
                              specifies that the sender is the media
                              source and expects the receiver to act
                              as a loopback mirror.
    Allowed attribute values: N/A
    Contact name:             Kaynam Hedayat
    Email address:            kh274@cornell.edu
    Telephone number:         +1-617-899-3279
    Attribute name:           loopback-mirror
    Type of attribute:        Media level.
    Subject to charset:       No.
    Purpose of attribute:     The 'loopback-mirror' attribute
                              specifies that the receiver will
                              mirror (echo) all received media back
                              to the sender of the RTP stream.
    Allowed attribute values: N/A

14.2. Media Types

 The IANA has registered the following media types.

14.2.1. audio/encaprtp

 To: ietf-types@iana.org
 Subject: Registration of media type audio/encaprtp
 Type name: audio
 Subtype name: encaprtp
 Required parameters:
    rate: RTP timestamp clock rate, which is equal to the sampling
    rate.  This is specified by the loopback source and reflected by
    the mirror.
 Optional parameters: N/A
 Encoding considerations: This media type is framed.

Kaplan, et al. Standards Track [Page 23] RFC 6849 SDP Media Loopback February 2013

 Security considerations: See Section 12 of RFC 6849.
 Interoperability considerations: N/A
 Published specification: RFC 6849.
 Applications that use this media type: Applications wishing to
    monitor and ensure the quality of transport to the edge of a given
    VoIP service.
 Additional information: N/A
 Contact: the authors of RFC 6849.
 Intended usage: LIMITED USE
 Restrictions on usage: This media type depends on RTP framing and
    hence is only defined for transfer via RTP.  Transfer within other
    framing protocols is not defined at this time.
 Author: Kaynam Hedayat.
 Change controller: IETF PAYLOAD working group delegated from
    the IESG.

14.2.2. video/encaprtp

 To: ietf-types@iana.org
 Subject: Registration of media type video/encaprtp
 Type name: video
 Subtype name: encaprtp
 Required parameters:
    rate: RTP timestamp clock rate, which is equal to the sampling
    rate.  This is specified by the loopback source and reflected by
    the mirror.
 Optional parameters: N/A
 Encoding considerations: This media type is framed.
 Security considerations: See Section 12 of RFC 6849.
 Interoperability considerations: N/A

Kaplan, et al. Standards Track [Page 24] RFC 6849 SDP Media Loopback February 2013

 Published specification: RFC 6849.
 Applications that use this media type: Applications wishing to
    monitor and ensure the quality of transport to the edge of a given
    Video Over IP service.
 Additional information: N/A
 Contact: the authors of RFC 6849.
 Intended usage: LIMITED USE
 Restrictions on usage: This media type depends on RTP framing and
    hence is only defined for transfer via RTP.  Transfer within other
    framing protocols is not defined at this time.
 Author: Kaynam Hedayat.
 Change controller: IETF PAYLOAD working group delegated from
    the IESG.

14.2.3. text/encaprtp

 To: ietf-types@iana.org
 Subject: Registration of media type text/encaprtp
 Type name: text
 Subtype name: encaprtp
 Required parameters:
    rate: RTP timestamp clock rate, which is equal to the sampling
    rate.  This is specified by the loopback source and reflected by
    the mirror.
 Optional parameters: N/A
 Encoding considerations: This media type is framed.
 Security considerations: See Section 12 of RFC 6849.
 Interoperability considerations: N/A
 Published specification: RFC 6849.

Kaplan, et al. Standards Track [Page 25] RFC 6849 SDP Media Loopback February 2013

 Applications that use this media type: Applications wishing to
    monitor and ensure the quality of transport to the edge of a given
    real-time text service.
 Additional information: N/A
 Contact: the authors of RFC 6849.
 Intended usage: LIMITED USE
 Restrictions on usage: This media type depends on RTP framing and
    hence is only defined for transfer via RTP.  Transfer within other
    framing protocols is not defined at this time.
 Author: Kaynam Hedayat.
 Change controller: IETF PAYLOAD working group delegated from
    the IESG.

14.2.4. application/encaprtp

 To: ietf-types@iana.org
 Subject: Registration of media type application/encaprtp
 Type name: application
 Subtype name: encaprtp
 Required parameters:
    rate: RTP timestamp clock rate, which is equal to the sampling
    rate.  This is specified by the loopback source and reflected by
    the mirror.
 Optional parameters: N/A
 Encoding considerations: This media type is framed.
 Security considerations: See Section 12 of RFC 6849.
 Interoperability considerations: N/A
 Published specification: RFC 6849.
 Applications that use this media type: Applications wishing to
    monitor and ensure the quality of transport to the edge of a given
    real-time application service.

Kaplan, et al. Standards Track [Page 26] RFC 6849 SDP Media Loopback February 2013

 Additional information: N/A
 Contact: the authors of RFC 6849.
 Intended usage: LIMITED USE
 Restrictions on usage: This media type depends on RTP framing and
    hence is only defined for transfer via RTP.  Transfer within other
    framing protocols is not defined at this time.
 Author: Kaynam Hedayat.
 Change controller: IETF PAYLOAD working group delegated from
    the IESG.

14.2.5. audio/rtploopback

 To: ietf-types@iana.org
 Subject: Registration of media type audio/rtploopback
 Type name: audio
 Subtype name: rtploopback
 Required parameters:
    rate: RTP timestamp clock rate, which is equal to the sampling
    rate.  This is specified by the loopback source and reflected by
    the mirror.
 Optional parameters: N/A
 Encoding considerations: This media type is framed.
 Security considerations: See Section 12 of RFC 6849.
 Interoperability considerations: N/A
 Published specification: RFC 6849.
 Applications that use this media type: Applications wishing to
    monitor and ensure the quality of transport to the edge of a given
    VoIP service.
 Additional information: N/A
 Contact: the authors of RFC 6849.

Kaplan, et al. Standards Track [Page 27] RFC 6849 SDP Media Loopback February 2013

 Intended usage: LIMITED USE
 Restrictions on usage: This media type depends on RTP framing and
    hence is only defined for transfer via RTP.  Transfer within other
    framing protocols is not defined at this time.
 Author: Kaynam Hedayat.
 Change controller: IETF PAYLOAD working group delegated from
    the IESG.

14.2.6. video/rtploopback

 To: ietf-types@iana.org
 Subject: Registration of media type video/rtploopback
 Type name: video
 Subtype name: rtploopback
 Required parameters:
    rate: RTP timestamp clock rate, which is equal to the sampling
    rate.  This is specified by the loopback source and reflected by
    the mirror.
 Optional parameters: N/A
 Encoding considerations: This media type is framed.
 Security considerations: See Section 12 of RFC 6849.
 Interoperability considerations: N/A
 Published specification: RFC 6849.
 Applications that use this media type: Applications wishing to
    monitor and ensure the quality of transport to the edge of a given
    Video Over IP service.
 Additional information: N/A
 Contact: the authors of RFC 6849.
 Intended usage: LIMITED USE

Kaplan, et al. Standards Track [Page 28] RFC 6849 SDP Media Loopback February 2013

 Restrictions on usage: This media type depends on RTP framing and
    hence is only defined for transfer via RTP.  Transfer within other
    framing protocols is not defined at this time.
 Author: Kaynam Hedayat.
 Change controller: IETF PAYLOAD working group delegated from
    the IESG.

14.2.7. text/rtploopback

 To: ietf-types@iana.org
 Subject: Registration of media type text/rtploopback
 Type name: text
 Subtype name: rtploopback
 Required parameters:
    rate: RTP timestamp clock rate, which is equal to the sampling
    rate.  This is specified by the loopback source and reflected by
    the mirror.
 Optional parameters: N/A
 Encoding considerations: This media type is framed.
 Security considerations: See Section 12 of RFC 6849.
 Interoperability considerations: N/A
 Published specification: RFC 6849.
 Applications that use this media type: Applications wishing to
    monitor and ensure the quality of transport to the edge of a given
    real-time text service.
 Additional information: N/A
 Contact: the authors of RFC 6849.
 Intended usage: LIMITED USE
 Restrictions on usage: This media type depends on RTP framing and
    hence is only defined for transfer via RTP.  Transfer within other
    framing protocols is not defined at this time.

Kaplan, et al. Standards Track [Page 29] RFC 6849 SDP Media Loopback February 2013

 Author: Kaynam Hedayat.
 Change controller: IETF PAYLOAD working group delegated from
    the IESG.

14.2.8. application/rtploopback

 To: ietf-types@iana.org
 Subject: Registration of media type application/rtploopback
 Type name: application
 Subtype name: rtploopback
 Required parameters:
    rate: RTP timestamp clock rate, which is equal to the sampling
    rate.  This is specified by the loopback source and reflected by
    the mirror.
 Optional parameters: N/A
 Encoding considerations: This media type is framed.
 Security considerations: See Section 12 of RFC 6849.
 Interoperability considerations: N/A
 Published specification: RFC 6849.
 Applications that use this media type: Applications wishing to
    monitor and ensure the quality of transport to the edge of a given
    real-time application service.
 Additional information: N/A
 Contact: the authors of RFC 6849.
 Intended usage: LIMITED USE
 Restrictions on usage: This media type depends on RTP framing and
    hence is only defined for transfer via RTP.  Transfer within other
    framing protocols is not defined at this time.

Kaplan, et al. Standards Track [Page 30] RFC 6849 SDP Media Loopback February 2013

 Author: Kaynam Hedayat.
 Change controller: IETF PAYLOAD working group delegated from
    the IESG.

15. Acknowledgements

 This document's editor would like to thank the original authors of
 the document: Kaynam Hedayat, Nagarjuna Venna, Paul E. Jones, Arjun
 Roychowdhury, Chelliah SivaChelvan, and Nathan Stratton.  The editor
 has made fairly insignificant changes in the end.  Also, we'd like to
 thank Magnus Westerlund, Miguel Garcia, Muthu Arul Mozhi Perumal,
 Jeff Bernstein, Paul Kyzivat, Dave Oran, Flemming Andreasen, Gunnar
 Hellstrom, Emil Ivov, and Dan Wing for their feedback, comments, and
 suggestions.

16. References

16.1. Normative References

 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3264]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
             with Session Description Protocol (SDP)", RFC 3264,
             June 2002.
 [RFC3550]   Schulzrinne, H., Casner, S., Frederick, R., and V.
             Jacobson, "RTP: A Transport Protocol for Real-Time
             Applications", STD 64, RFC 3550, July 2003.
 [RFC3551]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
             Video Conferences with Minimal Control", STD 65,
             RFC 3551, July 2003.
 [RFC3611]   Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
             "RTP Control Protocol Extended Reports (RTCP XR)",
             RFC 3611, November 2003.
 [RFC3711]   Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
             Norrman, "The Secure Real-time Transport Protocol
             (SRTP)", RFC 3711, March 2004.
 [RFC4566]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
             Description Protocol", RFC 4566, July 2006.

Kaplan, et al. Standards Track [Page 31] RFC 6849 SDP Media Loopback February 2013

 [RFC4961]   Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
             BCP 131, RFC 4961, July 2007.
 [RFC5234]   Crocker, D., Ed., and P. Overell, "Augmented BNF for
             Syntax Specifications: ABNF", STD 68, RFC 5234,
             January 2008.

16.2. Informative References

 [RFC5245]   Rosenberg, J., "Interactive Connectivity Establishment
             (ICE): A Protocol for Network Address Translator (NAT)
             Traversal for Offer/Answer Protocols", RFC 5245,
             April 2010.
 [RFC5389]   Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
             "Session Traversal Utilities for NAT (STUN)", RFC 5389,
             October 2008.
 [RFC5766]   Mahy, R., Matthews, P., and J. Rosenberg, "Traversal
             Using Relays around NAT (TURN): Relay Extensions to
             Session Traversal Utilities for NAT (STUN)", RFC 5766,
             April 2010.
 [RFC6263]   Marjou, X. and A. Sollaud, "Application Mechanism for
             Keeping Alive the NAT Mappings Associated with RTP / RTP
             Control Protocol (RTCP) Flows", RFC 6263, June 2011.

Kaplan, et al. Standards Track [Page 32] RFC 6849 SDP Media Loopback February 2013

Authors' Addresses

 Hadriel Kaplan (editor)
 Acme Packet
 100 Crosby Drive
 Bedford, MA  01730
 US
 EMail: hkaplan@acmepacket.com
 URI:   http://www.acmepacket.com
 Kaynam Hedayat
 EXFO
 285 Mill Road
 Chelmsford, MA  01824
 US
 EMail: kh274@cornell.edu
 URI:   http://www.exfo.com/
 Nagarjuna Venna
 Saperix
 c/o DogPatch Labs
 One Cambridge Center, 6th Floor
 Cambridge, MA  02142
 US
 EMail: vnagarjuna@saperix.com
 URI:   http://www.saperix.com/
 Paul E. Jones
 Cisco Systems, Inc.
 7025 Kit Creek Rd.
 Research Triangle Park, NC  27709
 US
 EMail: paulej@packetizer.com
 URI:   http://www.cisco.com/
 Nathan Stratton
 BlinkMind, Inc.
 2027 Briarchester Dr.
 Katy, TX  77450
 US
 EMail: nathan@robotics.net
 URI:   http://www.robotics.net/

Kaplan, et al. Standards Track [Page 33]

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