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  MMUSIC Working Group                                  H. Kaplan (ed.)
  Internet-Draft                                            Acme Packet
  Intended status: Proposed Standard                         K. Hedayat
  Expires: July 14, 2013                                           EXFO
                                                               N. Venna
                                                                Saperix
                                                               P. Jones
                                                    Cisco Systems, Inc.
                                                            N. Stratton
                                                        BlinkMind, Inc.
                                                       January 14, 2013
 
         An Extension to the Session Description Protocol (SDP)
        and Real-time Transport Protocol (RTP) for Media Loopback
                   draft-ietf-mmusic-media-loopback-27
 
 
 Status of this Memo
 
    This Internet-Draft is submitted to IETF in full conformance with
    the provisions of BCP 78 and BCP 79.
 
    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups. Note that
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    This Internet-Draft will expire on July 14, 2013.
 
 
 Copyright Notice
 
    Copyright (c) 2012 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
 
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    (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
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    This document may contain material from IETF Documents or IETF
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    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.
 
 
 Abstract
 
    The wide deployment of Voice over IP (VoIP), 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 SDP media types and
    attributes, which 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.
 
 
 Table of Contents
 
    1. Introduction..................................................3
       1.1 Use Cases Supported.......................................4
    2. Terminology...................................................6
 
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    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 NAT........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. Normative References........................................31
    17. 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, Text and Video over IP Services
    is 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.
 
 
 
 
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    The goal of active monitoring is to measure the media quality of a
    VoIP, 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., RTCP and RTCP-XR 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 RTP Control Protocol
    (RTCP) [RFC3550] and RTCP Extended Reports (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 and timestamps in the encapsulating
 
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    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
 
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    negotiation. The reencoded content is returned to the loopback
    source as an RTP packet with payload type corresponding to the
    reencoding format.
 
    This usage allows trouble-shooting at the codec level. The
    capability for path statistics is limited to what is available from
    RTCP reports.
 
 
 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 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 loop back for some media
 
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    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.
 
 
 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 which 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 loopack 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
 
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    ; 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"
 
    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 are as follows:
 
       a=loopback-source
 
    and
 
       a=loopback-mirror
 
 
 
 
 
 
 
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    Following is the Augmented BNF [RFC5234] for loopback-type:
 
    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.
 
    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 wish 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
 
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    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.
 
    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.
 
 
 
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    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
 
    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
 
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       a=rtpmap:112 encaprtp/8000
       a=rtpmap:113 rtploopback/8000
 
    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 NAT
 
    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.
 
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    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 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 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 rtp-pkt-loopback loopback type 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 number of frames/packet and
    bitrate of the encoded media according to the received media.
 
 
 7.  Payload formats for Packet loopback
 
 
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    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 loopback operation.  This is useful in cases where 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
    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: 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]).
 
    SSRC: set as described in RFC 3550 [RFC3550].
 
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    CC and CSRC fields are used as described in RFC 3550 [RFC3550].
 
 
 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]).
 
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    Fragmentation (F): 2 bits
 
    First Fragment (00) /Last Fragment (01) /No Fragmentation(10)/
    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, 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: Set to the value in the received packet.
 
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    Extension (X) bit: 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: set as described in RFC 3550 [RFC3550].
 
    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 direct loopback RTP
    format.
 
    m=audio 41352 RTP/AVP 112
    a=rtpmap:112 rtploopback/8000
 
 
 8.  SRTP Behavior
 
    Secure RTP [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, performs
    encryption/decryption, authentication, etc.  Therefore the loopback
    function on the mirror occurs after the SRTP packet has been
 
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    decrypted and authenticated, as a normal cleartext RTP packet
    without an MKI or authentication tag; once the 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 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 choose to support RTCP-XR, they SHOULD support RTCP-XR
    Loss Run Length Encoding (RLE) report block, Duplicate RLE report
    block, Statistics Summary report block, and VoIP Metric Reports
    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
 
 
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    transactions and are abbreviated and do not show the complete
    signaling for convenience.
 
 
 11.1    Offer for specific media loopback type
 
    An agent sends an SDP offer which 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 rtp-media-loopback loopback
    type.
 
    An answering agent sends an SDP answer which 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 is accepting 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 which 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
 
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    a=loopback-source
    a=rtpmap:0 pcmu/8000
    a=rtpmap:112 encaprtp/8000
    a=rtpmap:113 rtploopback/8000
 
 
    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 which looks like:
 
    v=0
    o=box 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 is accepting 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 which 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 which looks like:
 
    v=0
    o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
    s=-
    c=IN IP4 host.biloxi.example.com
 
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    t=0 0
    m=audio 0 RTP/AVP 0
    a=rtpmap:0 pcmu/8000
 
    Note in this case the answerer did not indicate loopback support,
    although it could have and still used a port number of 0 to
    indicate 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 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
 
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    loopback session, even if it is an authenticated session.  Devices
    that answer 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 light-weight 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
 
    [Note to RFC Editor: Please replace "XXXX" with the appropriate RFC
    number on publication]
 
 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:            kaynam.hedayat@exfo.com
       Telephone number:         +1-978-367-5611
       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 to 'loopback' may be
                                 one or more of "rtp-pkt-loopback" and
                                 "rtp-media-loopback". See Section 5
                                 of RFC XXXX for syntax.
 
       Contact name:             Kaynam Hedayat
       Email address:            kaynam.hedayat@exfo.com
 
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       Telephone number:         +1-978-367-5611
       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: None.
 
       Contact name:             Kaynam Hedayat
       Email address:            kaynam.hedayat@exfo.com
       Telephone number:         +1-978-367-5611
       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: None.
 
 
 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. The typical rate is 8000; other rates
                may be specified. This is specified by the loop back
                source, and reflected by the mirror.
 
           Optional parameters: none
 
 
 
           Encoding considerations: This media type is framed.
 
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           Security considerations: See Section 12 of RFC XXXX.
 
           Interoperability considerations: none
 
           Published specification: RFC XXXX.
 
           Applications which use this media type: Applications wishing
                to monitor and ensure the quality of transport to the
                edge of a given VoIP Service.
 
           Additional information: none
 
           Contact: the authors of RFC XXXX.
 
           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 loop back
                source, and reflected by the mirror.
 
           Optional parameters: none
 
           Encoding considerations: This media type is framed.
 
 
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           Security considerations: See Section 12 of RFC XXXX.
 
           Interoperability considerations: none
 
           Published specification: RFC XXXX.
 
           Applications which 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: none
 
           Contact: the authors of RFC XXXX.
 
           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 loop back
                source, and reflected by the mirror.
 
           Optional parameters: none
 
           Encoding considerations: This media type is framed.
 
           Security considerations: See Section 12 of RFC XXXX.
 
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           Interoperability considerations: none
 
           Published specification: RFC XXXX.
 
           Applications which 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: none
 
           Contact: the authors of RFC XXXX.
 
           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 loop back
                source, and reflected by the mirror.
 
           Optional parameters: none
 
           Encoding considerations: This media type is framed.
 
           Security considerations: See Section 12 of RFC XXXX.
 
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           Interoperability considerations: none
 
           Published specification: RFC XXXX.
 
           Applications which 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: none
 
           Contact: the authors of RFC XXXX.
 
           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. The typical rate is 8000; other rates
                may be specified. This is specified by the loop back
                source, and reflected by the mirror.
 
           Optional parameters: none
 
           Encoding considerations: This media type is framed.
 
           Security considerations: See Section 12 of RFC XXXX.
 
 
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           Interoperability considerations: none
 
           Published specification: RFC XXXX.
 
           Applications which use this media type: Applications wishing
                to monitor and ensure the quality of transport to the
                edge of a given VoIP Service.
 
           Additional information: none
 
           Contact: the authors of RFC XXXX.
 
           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 loop back
                source, and reflected by the mirror.
 
           Optional parameters: none
 
           Encoding considerations: This media type is framed.
 
           Security considerations: See Section 12 of RFC XXXX.
 
           Interoperability considerations: none
 
 
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           Published specification: RFC XXXX.
 
           Applications which 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: none
 
           Contact: the authors of RFC XXXX.
 
           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.
 
 
 14.2.7    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 loop back
                source, and reflected by the mirror.
 
           Optional parameters: none
 
           Encoding considerations: This media type is framed.
 
           Security considerations: See Section 12 of RFC XXXX.
 
           Interoperability considerations: none
 
           Published specification: RFC XXXX.
 
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           Applications which 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: none
 
           Contact: the authors of RFC XXXX.
 
           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.
 
 
 14.2.8    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 loop back
                source, and reflected by the mirror.
 
           Optional parameters: none
 
           Encoding considerations: This media type is framed.
 
           Security considerations: See Section 12 of RFC XXXX.
 
           Interoperability considerations: none
 
           Published specification: RFC XXXX.
 
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           Applications which 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: none
 
           Contact: the authors of RFC XXXX.
 
           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.
 
 
 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.  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 the 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.
 
 
 
 
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 Internet-Draft            SDP Media Loopback              January 2013
 
       [RFC3551] Schulzrinne, H., Casner, S., "RTP Profile for Audio
                  and Video Conferences with Minimial Control", STD 65,
                  RFC 3551, July 2003.
 
       [RFC3611] Almeroth, K., Caceres, R., Clark, A., Cole, R.,
                  Duffield, N., Friedman, T., Hedayat, K., Sarac, K.
                  and M. Westerlund, "RTP Control Protocol Extended
                  Reports (RTCP XR)", RFC 3611, November 2003.
 
       [RFC3711] Baugher, M., et al, "The Secure Real-time Transport
                  Protocol (SRTP)", RFC 3711, March 2004.
 
       [RFC4566] Handley, M., Jacobson, V., Perkins, C., "SDP: Session
                  Description Protocol", RFC 4566, July 2006.
 
       [RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol
                  (RTCP)", RFC 4961, July 2007.
 
       [RFC5234] Crocker, P. Overell, "Augmented ABNF for Syntax
                  Specification: ABNF", RFC 5234, October 2005.
 
 
 
 17.  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.
 
       [RFC6263] Marjou, X., Sollaud, A., "Application Mechanism for
                  Keeping Alive the NAT Mappings Associated with RTP /
                  RTP Control Protocol (RTCP) Flows", RFC 6263, June
                  2011.
 
 
 Authors' Addresses
 
 
       Hadriel Kaplan
       Acme Packet
       100 Crosby Drive
       Bedford, MA  01730
       USA
 
       EMail: hkaplan@acmepacket.com
       URI:   http://www.acmepacket.com
 
 
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 Internet-Draft            SDP Media Loopback              January 2013
 
       Kaynam Hedayat
       EXFO
       285 Mill Road
       Chelmsford, MA  01824
       US
 
       EMail: kaynam.hedayat@exfo.com
       URI:   http://www.exfo.com/
 
 
       Nagarjuna Venna
       Saperix
       738 Main Street, #398
       Waltham, MA 02451
       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
 
       EMail: nathan@robotics.net
       URI:   http://www.robotics.net/
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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