K. Hedayat 
  Internet Draft                                          Brix Networks 
  Expires: June 30, 2005                                       P. Jones 
                                                    Cisco Systems, Inc. 
                                                        A. Roychowdhury 
                                                Hughes Software Systems 
                                                         C. SivaChelvan 
                                                    Cisco Systems, Inc. 
                                                            N. Stratton 
                                                             BroadVoice 
                                                       December 2, 2004 
  
     An Extension to the Session Description Protocol (SDP) for Media 
                                Loopback 
		draft-ietf-mmusic-media-loopback-00.txt
                                      
  
 Status of this Memo  
     
     
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    accordance with RFC 3668 (BCP 79). 
     
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    Section 3 of RFC 3667 (BCP 78). 
     
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 Abstract 
     
  
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    The wide deployment of VoIP and Video over IP services has 
    introduced new challenges in managing and maintaining voice/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 or Video over IP service.  Today in 
    networks that deliver real-time media, short of running ' ping' and 
    ' traceroute' to the edge, service providers 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 attributes which enables 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 and Video 
    Over IP performance metrics. 
     
     
 Table of Contents 
     
     
    1. Introduction..................................................3 
    2. Terminology...................................................3 
    3. Offering Entity Behavior......................................3 
    4. Answering Entity Behavior.....................................4 
    5. SDP Constructs Syntax.........................................4 
       5.1 Loopback Type Attribute...................................4 
       5.2 Loopback Mode Attribute...................................5 
       5.3 Generating the Offer for Loopback Session.................5 
       5.4 Generating the Answer for Loopback Session................6 
       5.5 Offerer Processing of the Answer..........................7 
       5.6 Modifying the Session.....................................7 
    6. RTP Requirements..............................................7 
    7. RTCP Requirements.............................................8 
    8. Examples......................................................8 
       8.1 Offer for specific media loopback type....................8 
       8.2 Offer for choice of media loopback type...................9 
       8.3 Response to INVITE request rejecting loopback media......10 
    9. Implementer Guidelines.......................................11 
    10. Security Considerations.....................................11 
    11. IANA Considerations.........................................11 
    12. Acknowledgements............................................11 
    13. References..................................................11 
       13.1 Normative References....................................11 
     


  
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 1. Introduction 
  
  
    The overall quality, reliability, and performance of VoIP and Video 
    over IP services relies 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 VoIP 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 pro-actively managing the performance and 
    quality of VoIP based services. 
     
    The goal of active monitoring is to measure the media quality of a 
    VoIP 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 memo introduces new SDP media 
    attributes that enable establishment of media sessions where the 
    media is looped back to the transmitter.  The offer/answer model 
    per RFC 3264 [RFC3264] is used to establish a loopback connection.  
    Furthermore, this extension provides guidelines on handling RTP 
    (RFC 3550) [RFC3550], as well as usage of RTCP (RFC 3550) [RFC3550] 
    and RTCP XR (RFC 3611) [RFC3611] for reporting media related 
    measurements. 
     
     
 2. Terminology 
  
  
    In this document, the key words "MUST", "MUST NOT", "REQUIRED", 
    "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 
    and "OPTIONAL" are to be interpreted as described in RFC 2119 
    [RFC3264] and indicate requirement levels for compliant 
    implementations. 
     
     
 3. Offering Entity Behavior 
     
     


  
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    An offering entity 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 offering entity MUST look for the 
    "loopback" media attributes in the media description(s) of the 
    response from the answering entity for confirmation that the 
    request is accepted. 
     
     
 4. Answering Entity Behavior 
  
     
    An answering entity 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 
    response.  The server MAY reject the "loopback" request for 
    specific media types as defined in section 5.4.1 of this 
    specification.  
     
     An answering entity that is not compliant to this specification 
     and which receives an offer with the "loopback" media attributes 
     MAY safely ignore the attribute and treat the incoming offer as a 
     normal request. 
         
  
 5. SDP Constructs Syntax 
  
  
    Two new media attributes are defined: one indicates the type of 
    loopback and one indicates the mode of the loopback. 
     
     
 5.1 Loopback Type Attribute 
     
     
    The loopback type is a property media attribute with the following 
    syntax:  
     
       a=loopback:<loopback-type> 
     
    Following is the Augmented BNF (RFC 2234) [RFC2234] for loopback-
    type: 
     
    loopback-type = loopback-type-choice [ space loopback-type-choice ] 
    loopback-type-choice = "rtp-pkt-loopback" | "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. 
  
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    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 overheads 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. 
     
    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. 
     
     
 5.2 Loopback Mode Attribute 
     
     
    The loopback mode is a value media attribute that is used to 
    indicate the mode of the loopback.  These attributes can be viewed 
    as additional mode attributes similar to sendonly, recvonly, etc.  
    The syntax of the loopback mode media attribute is: 
     
       a=<loopback-mode> 
     
    The loopback-mode values are loopback-source and loopback-mirror.. 
     
    loopback-source: This attribute specifies that the sender is the 
    media source and expects the receiver to act as a loopback-mirror. 
     
    loopback-mirror: This attribute specifies that the receiver will 
    mirror (echo) all received media back to the sender of the RTP 
    stream.  No media is generated locally by the reciver for 
    transmission in the mirrored stream. 
     
     
 5.3 Generating the Offer for Loopback Session 
     
     
    If an offerer wishes to make a loopback request, it MUST include 
    both the loopback-type and loopback-mode attribute in a valid SDP 
    offer: 
     
    Example:   a=loopback-type:rtp-media-loopback 
               a=loopback-source 
     



  
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    Note: A loopback offer in a given media description MUST NOT 
    contain the standard mode attributes sendonly, recvonly, sendrecv 
    or inactive. 
     
    The offerer may offer more than one loopback-type in the SDP offer. 
    In this case the answer MUST include only one of the loopback types 
    that is accepted by the answerer.  The answerer SHOULD give 
    preference to the first loopback-type in the SDP offer. 
     
    For loopback-source media (e.g. audio) streams, the port number and 
    the address in the offer indicates where the offerer would like to 
    receive the media stream.  The payload type numbers indicate the 
    value of the payload the offerer expects to receive, and would 
    prefer to send.  However, the answer might indicate a different 
    payload type number for the same codec.  In that case, the offerer 
    MUST send the payload type received in the answer. 
     
     
  
 5.4 Generating the Answer for Loopback Session 
     
     
    If an answerer wishes to accept the loopback request it MUST 
    include both the loopback mode and loopback type attribute in the 
    answer. If a stream is offered with loopback-source or 
    loopback-mirror attributes, the corresponding stream MUST be 
    loopback-mirror or loopback-source respectively, provided that 
    answerer is capable of supporting the requested loopback-type.   
     
    For example, if the offer contains: 
     
       a=loopback-type:rtp-media-loopback 
       a=loopback-source 
     
    The answer that is capable of supporting the offer MUST contain: 
     
       a=loopback-type:rtp-media-loopback  
       a=loopback-mirror 
     
    As previously stated if a stream is offered with multiple loopback 
    type attributes, the corresponding stream MUST contain only one 
    loopback type attribute selected by the answerer. 
     
    For example, if the offer contains: 
     
       a=loopback-type:rtp-media-loopback rtp-pkt-loopback 
       a=loopback-source 
     


  
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    The answer that is capable of supporting the offer and chooses to 
    loopback the media using the rtp-media-loopback type MUST contain: 
     
       a=loopback-type:rtp-media-loopback  
       a=loopback-mirror 
     
     
  
 5.4.1 Rejecting the Loopback Offer 
     
     
    An offered stream with loopback-source MAY be rejected, 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 may be rejected by setting the media 
    port number to zero, according to RFC 3264 [RFC3264], in the 
    answer. 
     
     
 5.5 Offerer Processing of the Answer 
     
     
    The answer to a loopback-source MUST be loopback-mirror.  The 
    answer to a loopback-mirror MUST be loopback-source.  In addition, 
    the "m=" line MUST contain at least one codec that the answerer is 
    willing to both send and receive. 
     
    If the answer does not contain a=loopback-mirror or 
    a=loopback-source or contains any other standard mode attributes, 
    it is assumed that the loopback extensions are not supported by the 
    target UA. 
     
     
 5.6 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.  In case of SIP this is defined in section 8 of RFC 3264 
    [RFC3264].  This also includes transitioning from a normal media 
    processing mode to loopback mode, and vice a versa. 
     
     
 6. RTP Requirements 
     
     
    An answering entitity that is compliant to this specification and 
    accepting a media with rtp-pkt-loopback loopback-type MUST 
    re-generate all of the RTP header fields as it does when 
  
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    transmitting other media.  However, the answering entity MUST 
    maintain the timing information of the received RTP packets when 
    generating the RTP timestamp for the transmit packets.  Maintaining 
    the timing information of the RTP packets enables the offerer to 
    re-construct the incoming media and take account for impairments 
    from gaps in the media due to packet loss.  Note that RTP Sequence 
    numbers are re-generated by the UAS and will not provide packet 
    loss information to the receiver of the loopback media. 
     
    An answering entity that is compliant to this specification and 
    accepting a media with rtp-media-loopback loopback-type MUST 
    transmit all received media back to the sender . 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 answering entity 
    MUST re-generate all the RTP header fields as it would when 
    transmitting media. The UAS MAY choose to encode the loopback media 
    according to any of the media descriptions supported by the UAC. 
    Furthermore, in cases where the same media type is looped back, the 
    UAS MAY choose to preserve number of frames/packet and bitrate of 
    the encoded media according to the received media. 
     
         
 7. 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 entity that is 
    compliant to this specification SHOULD support RTCP per [RFC3550] 
    and RTCP-XR per RFC 3611 [RFC3611].  Furthermore, if the client or 
    the server choose to support RTCP-XR,  they SHOULD support RTCP-XR 
    Statistics Summary Report Block and VoIP Metric Reports Block per 
    sections 4.6 and 4.7 of  RFC 3611 [RFC3611].  The client and the 
    server MAY support other RTCP-XR reporting blocks as defined by RFC 
    3611 [RFC3611]. 
     
     
 8. 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. 
     
     
 8.1 Offer for specific media loopback type 
     
  
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    A client sends an INVITE request with SDP which looks like: 
     
    v=0 
    o=user1 2890844526 2890842807 IN IP4 126.16.64.4 
    s=Example 
    i=An example session 
    e=user@example.com 
    c=IN IP4 224.2.17.12/127 
    t=0 0 
    m=audio 49170 RTP/AVP 0 
    a=loopback:rtp-media-loopback 
    a=loopback-source 
     
    The client is offering to source the media and expects the server 
    to mirror the RTP stream per rtp-media-loopback loopback type. 
     
    A server sends a response with SDP which looks like: 
     
    v=0 
    o=user1 2890844526 2890842807 IN IP4 126.16.64.4 
    s=Example 
    i=An example session 
    e=user@example.com 
    c=IN IP4 224.2.17.12/127 
    t=0 0 
    m=audio 49170 RTP/AVP 0 
    a=loopback:rtp-media-loopback 
    a=loopback-mirror 
     
    The server is accepting to mirror the media from the client at the 
    media level. 
     
     
 8.2 Offer for choice of media loopback type 
     
     
    A client sends an INVITE request with SDP which looks like: 
     
    v=0 
    o=user1 2890844526 2890842807 IN IP4 126.16.64.4 
    s=Example 
    i=An example session 
    e=user@example.com 
    c=IN IP4 224.2.17.12/127 
    t=0 0 
    m=audio 49170 RTP/AVP 0 
    a=loopback:rtp-media-loopback rtp-pkt-loopback 
    a=loopback-source 
  
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    The client is offering to source the media and expects the server 
    to mirror the RTP stream at either the media or rtp level. 
     
    A server sends a response with SDP which looks like: 
     
    v=0 
    o=user1 2890844526 2890842807 IN IP4 126.16.64.4 
    s=Example 
    i=An example session 
    e=user@example.com 
    c=IN IP4 224.2.17.12/127 
    t=0 0 
    m=audio 49170 RTP/AVP 0 
    a=loopback:rtp-pkt-loopback 
    a=loopback-mirror 
     
    The server is accepting to mirror the media from the client at the 
    packet level. 
     
     
 8.3 Response to INVITE request rejecting loopback media 
     
     
    A client sends an INVITE request with SDP which looks like: 
     
    v=0 
    o=user1 2890844526 2890842807 IN IP4 126.16.64.4 
    s=Example 
    i=An example session 
    e=user@example.com 
    c=IN IP4 224.2.17.12/127 
    t=0 0 
    m=audio 49170 RTP/AVP 0 
    a=loopback:rtp-media-loopback 
    a=loopback-source 
     
    The client is offering to source the media and expects the server 
    to mirror the RTP stream at the media level. 
     
    A server sends a response with SDP which looks like: 
     
    v=0 
    o=user1 2890844526 2890842807 IN IP4 126.16.64.4 
    s=Example 
    i=An example session 
    e=user@example.com 
    c=IN IP4 224.2.17.12/127 
    t=0 0 
  
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    m=audio 0 RTP/AVP 0 
    a=loopback:rtp-media-loopback 
    a=loopback-mirror 
     
    NOTE: Loopback request may be rejected by either not including the 
    loopback mode attribute(for backward compatibility) or setting the 
    media port number to zero, or both, in the response. 
     
     
 9. Implementer Guidelines 
     
     
    This section provides guidelines to the implementers of this 
    extension. 
     
     
 10. Security Considerations 
     
     
    The security considerations of [RFC3261] apply. Furthermore, given 
    that media loopback may be automated without the end userÆs 
    knowledge, the server of the media loopback should be aware of 
    denial of service attacks. It is recommended that sessions with 
    media loopback are authenticated and the frequency of such sessions 
    are limited by the server.  
     
     
 11. IANA Considerations 
     
     
    There are no IANA considerations associated with this 
    specification. 
  
  
 12. Acknowledgements 
     
     
    The authors wish to thank Flemming Andreasen, Jeff Bernstein, Paul 
    Kyzivat, and Dave Oran for their comments and suggestions. 
     
     
 13. References  
  
  
 13.1 Normative References 
  
  
       [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., 
                  Johnston, A., Peterson, J., Sparks, R., Handley, M. 
  
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                  and E. Schooler, "SIP: Session Initiation Protocol", 
                  RFC 3261, STD 1, June 2002. 
     
     
       [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer 
                  Model with the Session Description Protocol (SDP)", 
                  RFC 3264, STD 1, June 2002. 
     
     
       [RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. 
                  Jacobson, "RTP: A Transport Protocol for Real-Time 
                  Applications", RFC 3550, STD 1, 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, STD 1, November 2003.  
     
     
       [RFC2234] Crocker, P. Overell, "Augmented ABNF for Syntax 
                  Specification: ABNFö, RFC 3611, STD 1, November 1997. 
  
  
    Authors' Addresses 
     
     
       Kaynam Hedayat 
       Brix Networks 
       285 Mill Road 
       Chelmsford, MA  01824 
       US 
     
       Phone: +1 978 367 5611 
       EMail: khedayat@brixnet.com 
       URI:   http://www.brixnet.com/ 
     
     
       Paul E. Jones 
       Cisco Systems, Inc. 
       7025 Kit Creek Rd. 
       Research Triangle Park, NC  27709 
       US 
     
       Phone: +1 919 392 6948 
       EMail: paulej@packetizer.com 
       URI:   http://www.cisco.com/ 
        
     
  
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       Arjun Roychowdhury 
       Hughes Software Systems 
       11717 Exploration Lane 
       Germantown, MD  20876 
       US 
     
       Phone: +1 301 212 7860 
       EMail: aroychow@hssworld.com 
       URI:   http://www.hssworld.com/ 
     
     
       Chelliah SivaChelvan 
       Cisco Systems, Inc. 
       2200 East President George Bush Turnpike 
       Richardson, TX  75082 
       US 
     
       Phone: +1 972 813 5224 
       EMail: chelliah@cisco.com 
       URI:   http://www.cisco.com/ 
     
     
       Nathan Stratton 
       BroadVoice 
       900 Chelmsford Street 
       Tower Three 
       Lowell, MA  01851 
       US 
     
       Phone: +1 978 418 7320 
       EMail: nstratton@broadvoice.com 
       URI:   http://www.broadvoice.com/ 
     
     
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    assurances of licenses to be made available, or the result of an 
    attempt made to obtain a general license or permission for the use 
  
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    of such proprietary rights by implementers or users of this 
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    Funding for the RFC Editor function is currently provided by the 
    Internet Society. 
     









  
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