An Extension to the Session Description Protocol (SDP) and Real-time Transport Protocol (RTP) for Media Loopback
draft-ietf-mmusic-media-loopback-19

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Document Type Active Internet-Draft (mmusic WG)
Authors Hadriel Kaplan  , Kaynam Hedayat  , Nagarjuna Venna  , Paul Jones  , Nathan Stratton 
Last updated 2012-07-12
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Internet Draft                                        H. Kaplan (ed.) 
  Expires: December 12, 2012                                Acme Packet 
                                                             K. Hedayat 
                                                                   EXFO 
                                                               N. Venna 
                                                                Saperix 
                                                               P. Jones 
                                                    Cisco Systems, Inc. 
                                                            N. Stratton 
                                                        BlinkMind, Inc. 
                                                          July 12, 2012 
  
         An Extension to the Session Description Protocol (SDP)  
        and Real-time Transport Protocol (RTP) for Media Loopback 
                   draft-ietf-mmusic-media-loopback-19 
                                      
  
 Status of this Memo
     
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    This Internet-Draft will expire on December 12, 2012. 
      
  
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    Copyright (c) 2012 IETF Trust and the persons identified as the 
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    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 
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 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 
    3. Overview of Operation.........................................6 
       3.1 SDP Offerer Behavior......................................6 
       3.2 SDP Answerer Behavior.....................................6 
    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.....................11 
       5.4 Modifying the Session....................................12 
  
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       5.5 Establishing Sessions Between Entities Behind NAT........12 
    6. RTP Requirements.............................................12 
    7. Payload formats for Packet loopback..........................13 
       7.1 Encapsulated Payload format..............................13 
       7.2 Direct loopback RTP payload format.......................16 
    8. SRTP Behavior................................................17 
    9. RTCP Requirements............................................17 
    10. Congestion Control..........................................18 
    11. Examples....................................................18 
       11.1 Offer for specific media loopback type..................18 
       11.2 Offer for choice of media loopback type.................19 
       11.3 Answerer rejecting loopback media.......................20 
    12. Security Considerations.....................................20 
    13. Implementation Considerations...............................21 
    14. IANA Considerations.........................................22 
       14.1 SDP Attributes..........................................22 
       14.2 Media Types.............................................22 
    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. 
     
    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 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 

  
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    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 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 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 

  
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    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 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. 
     
     

  
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 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. 
     
    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 memo and attempting to establish a 
    media session with media loopback MUST include "loopback" media 
    attributes for each individual media description in the offer 
    message.  The offerer MUST look for the "loopback" media attributes 
    in the media description(s) of the response from the answer for 
    confirmation that the request is accepted. 
     
     
 3.2    SDP Answerer Behavior 
     
    An SDP answerer compliant to this specification and receiving an 
    offer containing media descriptions with the "loopback" media 
    attributes MUST acknowledge the request by including the received 
    "loopback" media attributes for each media description in its 
    answer if it agrees to do the loopback. If the answerer does not 
    want to do loopback or wants to reject the "loopback" request for 
  
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    specific media descriptions, it MUST do so as defined in this 
    section.  
     
    An answerer MAY 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 
    MUST be 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: 
     
    loopback-attr          = "a=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 
  
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    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 MUST 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 
     
     
    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 MUST include 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. 
     
  

  
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 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 MUST include 
    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 
    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 later sections 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 
  
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    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 MUST follow SDP 
    offer/answer rules for the direction attribute, but directions of 
    "sendonly" or "recvonly" do not apply for loopback operation and 
    hence MUST NOT be used. 
     
    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. 
     
    If an answerer wishes to accept the loopback request it MUST 
    include both the loopback role and loopback type attributes in the 
    answer. When a stream is offered with the loopback-source 
    attribute, the corresponding stream in the response MUST 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 

  
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    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 
     
    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 
     
  
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    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 
     
    ICE/STUN/TURN provide a general solution to establishing media 
    sessions between entities that are behind NATs, as defined in 
    [RFC5245]. Loopback sessions that involve one or more endpoints 
    behind NATs SHOULD use these general solutions wherever possible. 
     
    Furthermore, 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] SHOULD 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.  In other words both agents MUST 
    send packets from the source address and port they receive packets 
    on. 
     
     
 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 MUST loopback the 
    incoming RTP packets using either the encapsulated RTP payload 

  
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    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 MUST be treated as if it were to be played (e.g. the 
    media stream MAY receive treatment from PLC algorithms).  The 
    mirroring entity MUST re-generate 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 MAY choose to preserve number of frames/packet and bitrate 
    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 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 
    MUST be encapsulated in a separate encapsulating RTP packet; the 
    encapsulated packet MUST 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 

  
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    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]. 
     
    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 MUST be 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 MUST be 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. 

  
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     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             | 
    |                             ....                              | 
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 
     
    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 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]). 
     
    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 
  
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    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 MUST loop 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. 
     
    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].  
  
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 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 
    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. 
     
     
 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 RLE report block, Duplicate RLE report block, Statistics 
  
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    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.  
     
    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 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. 
  
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    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 
    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 
  
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    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 
    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 

  
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    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 
    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. 
     
  
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 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 
    <kaynam.hedayat@exfo.com>. 
       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 this document for syntax. 
     
       Contact name:             Kaynam Hedayat 
    <kaynam.hedayat@exfo.com>. 
       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 
    <kaynam.hedayat@exfo.com>. 
       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: 
     
  
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 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. 
     
           Security considerations: See Section 12 of this document. 
     
           Interoperability considerations: none 
     
           Published specification: This document. 
     
           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 this document. 
     
           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 Audio/Video Transport working 
                group delegated from the IESG. 
  
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 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. 
     
           Security considerations: See Section 12 of this document. 
     
           Interoperability considerations: none 
     
           Published specification: This document. 
     
           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 this document. 
     
           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 Audio/Video Transport working 
                group delegated from the IESG. 
     
  
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 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 this document. 
     
           Interoperability considerations: none 
     
           Published specification: This document. 
     
           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 this document. 
     
           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 Audio/Video Transport working 
                group delegated from the IESG. 
     
     
  
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 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 this document. 
     
           Interoperability considerations: none 
     
           Published specification: This document. 
     
           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 this document. 
     
           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 Audio/Video Transport working 
                group delegated from the IESG. 
     

  
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 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 this document. 
     
           Interoperability considerations: none 
     
           Published specification: This document. 
     
           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 this document. 
     
           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 Audio/Video Transport working 
                group delegated from the IESG. 
     

  
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 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 this document. 
     
           Interoperability considerations: none 
     
           Published specification: This document. 
     
           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 this document. 
     
           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 Audio/Video Transport working 
                group delegated from the IESG. 
     
     

  
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 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 this document. 
     
           Interoperability considerations: none 
     
           Published specification: This document. 
     
           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 this document. 
     
           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 Audio/Video Transport working 
                group delegated from the IESG. 
     
     

  
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 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 this document. 
     
           Interoperability considerations: none 
     
           Published specification: This document. 
     
           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 this document. 
     
           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 Audio/Video Transport working 
                group delegated from the IESG. 
     
     
  
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 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, Flemming 
    Andreasen, Gunnar Hellstrom, Emil Ivov and Dan Wing for their 
    feedback, comments and suggestions. 
     
 16.   Normative References  
  
  
       [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. 
     
     
       [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.  
     
     
       [RFC5234] Crocker, P. Overell, "Augmented ABNF for Syntax 
                  Specification: ABNF", RFC 5234, October 2005. 
  
       [RFC2119] Bradner, S.,"Key words for use in RFCs to Indicate 
                  Requirement Levels", BCP 14, RFC 2119, March 1997. 
  
  
       [RFC2736] Handley, M., Perkins, C., "Guidelines for Writers of 
                  RTP Payload Format Specifications", RFC 2736, BCP 
                  0036, December 1999. 
  
  
       [RFC3551] Schulzrinne, H., Casner, S., "RTP Profile for Audio 
                  and Video Conferences with Minimial Control", STD 65, 
                  RFC 3551, July 2003. 
  
       [RFC4566] Handley, M., Jacobson, V., Perkins, C., "SDP: Session 
                  Description Protocol", RFC 4566, July 2006. 
     

  
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       [RFC4855] Casner, S., "Media Type Registration of RTP Payload 
                  Formats", RFC 4855, February 2007. 
     
     
 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 
     
       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/ 
     
  
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       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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