K. Hedayat
  Internet Draft                                          Brix Networks
  Expires: December 27,2005 May 11, 2006                                        P. Jones
                                                    Cisco Systems, Inc.
                                                        A. Roychowdhury
                                           Flextronics Software Systems
                                                         C. SivaChelvan
                                                    Cisco Systems, Inc.
                                                            N. Stratton
                                                             BroadVoice
                                                          June 27,
                                                       November 7, 2005

     An Extension to the Session Description Protocol (SDP) for Media
                                Loopback
			draft-ietf-mmusic-media-loopback-01.txt
                   draft-ietf-mmusic-media-loopback-02

 Status of this Memo

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    applicable patent or other IPR claims of which he or she is aware
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 Copyright Notice

    Copyright (C) The Internet Society (2005).

 Abstract

    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......................................4
    4. Answering Entity Behavior.....................................4
    5. SDP Constructs Syntax.........................................4
       5.1 Loopback Type Attribute...................................4
       5.2 Loopback Mode Attribute...................................6
       5.3 Generating the Offer for Loopback Session.................6
       5.4 Generating the Answer for Loopback Session................7
       5.5 Offerer Processing of the Answer..........................8
       5.6 Modifying the Session.....................................8
    6. RTP Requirements..............................................8
    7. RTCP Requirements.............................................9
    8. Examples......................................................9
       8.1 Offer for specific media loopback type....................9
       8.2 Offer for choice of media loopback type..................10
       8.3 Offer for choice of media loopback type with
       rtp-start-loopback...........................................11
       8.4 Response to INVITE request rejecting loopback media......12
       8.5 Response to INVITE request rejecting loopback media with
       rtp-start-loopback...........................................13
    9. Security Considerations......................................13
    10. IANA Considerations.........................................14
    11. Acknowledgements............................................14
    12. References..................................................14
       12.1 Normative References....................................14

 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

    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 |
    rtp-start-loopback"

    The loopback type is used to indicate the type of loopback.  The
    loopback-type values are rtp-pkt-loopback, rtp-media-loopback, and
    rtp-start-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 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.

    rtp-start-loopback: In certain scenarios it is possible that the
    media transmitted by the offering entity is blocked by a network
    element until the answering entity starts transmitting packets.
    One example of this scenario is the presence of an RTP relay in the
    path of the media.  RTP relays exist in VoIP networks for purpose
    of NAT and Firewall traversal.  If an RTP relay is present the
    offering entityÆs entity’s packets are dropped by the RTP relay until the
    answering entity has started transmitting media and the media state
    within the RTP relay is established.  This loopback attribute is
    used to specify the media type for transmitting media packets by
    the answering entity prior to the loopback process for the purpose
    of setting media state within the network.  In the presence of this
    loopback attribute the answering entity will transmit media,
    according to the description that contains this attribute, until it
    receives media from the offering entity.  After the first media
    packet is received from the offering entity, the answering entity
    MUST terminate the transmission of rtp-start-loopback media and
    MUST start looping back media as defined by the other loopback
    attributes present in the offer.  If an offer includes the
    rtp-start-loopback attribute it MUST also include at least one
    other attribute as defined in this section.  The offering entity is
    able to filter rtp-start-loopback packets from other types of
    loopback with the payload type of the packet. The media port number
    for rtp-start-loopback MUST be the same as the corresponding
    loopback attribute that will take over after the reception of first
    media packet from the offering entity.

    It is recommended that an offering entity specifying media with
    either rtp-pkt-loopback or rtp-media-loopback attribute also
    specify the rtp-start-loopback attribute unless the offering entity
    is certain that its media will not be blocked by a network entity
    as explained above.

 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.

    The loopback mode attribute does not apply to rtp-start-loopback
    attribute and MAY be omitted.  If the loopback mode attribute is
    present with the rtp-start-loopback attribute it MUST be ignored if received by the answering entity. entityt.

 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:rtp-media-loopback
               a=loopback-source

    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:rtp-media-loopback
       a=loopback-source

    The answer that is capable of supporting the offer MUST contain:

       a=loopback: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:rtp-media-loopback rtp-pkt-loopback
       a=loopback-source

    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:rtp-media-loopback
       a=loopback-mirror
 5.4.1 Rejecting the Loopback Offer

    An offered stream with loopback-source MAY be rejected, 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], zero in the
    answer. answer as per RFC 3264 [RFC3264].

 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
    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 answering entity 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 answering entity MAY choose to encode the
    loopback media according to any of the media descriptions supported
    by the UAC. offering entity. Furthermore, in cases where the same media
    type is looped back, the
    UAS answering entity 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
    Loss RLE report block, Duplicate RLE report block, Statistics
    Summary Report Block report block, and VoIP Metric Reports Block per sections 4.6
    4.1, 4.2, 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 SIP-based
    transactions and are abbreviated and do not show the complete
    signaling for convenience.

 8.1 Offer for specific 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
    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

    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 Offer for choice of media loopback type with rtp-start-loopback

    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
    m=audio 49170 RTP/AVP 100
    a=loopback:rtp-start-loopback

    The client is offering to source the media and expects the server
    to mirror the RTP stream at either the media or rtp level.  The
    client also expects the server to source media until it receives
    packets from the server per media described with the
    rtp-start-loopback attribute.

    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
    m=audio 49170 RTP/AVP 100
    a=rtpmap:100 pcmu/8000
    a=loopback:rtp-start-loopback

    The server is accepting to mirror the media from the client at the
    packet level.  The server is also accepting to source media until
    it receives media packets from the client.

 8.4 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
    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.

 8.5 Response to INVITE request rejecting loopback media with
     rtp-start-loopback

    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
    m=audio 49170 RTP/AVP 100
    a=loopback:rtp-start-loopback

    The client is offering to source the media and expects the server
    to mirror the RTP stream at the media level.  The client also
    expects the server to source media until it receives packets from
    the server per media described with the rtp-start-loopback
    attribute.

    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 0 RTP/AVP 0
    a=loopback:rtp-media-loopback
    a=loopback-mirror
    m=audio 0 RTP/AVP 0
    a=loopback:rtp-start-loopback

    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. Security Considerations
    The security considerations of [RFC3261] apply. Furthermore, given
    that media loopback may be automated without the end user's
    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.

 10. IANA Considerations

    There are no IANA considerations associated with this
    specification.

 11. Acknowledgements

    The authors wish to thank Flemming Andreasen, Jeff Bernstein, Paul
    Kyzivat, and Dave Oran for their comments and suggestions.

 12. References

 12.1 Normative References

       [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
                  Johnston, A., Peterson, J., Sparks, R., Handley, M.
                  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, ABNF”, 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/

       Arjun Roychowdhury
       Flextronics Software Systems
       11717 Exploration Lane
       Germantown, MD  20876
       US

       Phone: +1 301 212 7860
       EMail: arjun.roy@flextronicssoftware.com
       URI:   http://www.flextronicssoftware.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 410 908 7587
       EMail: nstratton@broadvoice.com nathan@robotics.net
       URI:   http://www.broadvoice.com/

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