K. Hedayat
  Internet Draft                                          Brix Networks
  Expires: May 11, December 2006                                       P. Jones
                                                    Cisco Systems, Inc.
                                                        A. Roychowdhury
                                           Flextronics Software Systems
                                                                 Hughes
                                                         C. SivaChelvan
                                                    Cisco Systems, Inc.
                                                            N. Stratton
                                                             BroadVoice
                                                       November 7, 2005
     An Extension to the Session Description Protocol (SDP) for Media
                                Loopback
                   draft-ietf-mmusic-media-loopback-02
                   draft-ietf-mmusic-media-loopback-03

 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). (2006).

 Abstract

    The wide deployment of VoIP Voice over IP (VoIP), Real-time Text and
    Video over IP services has introduced new challenges in managing
    and maintaining voice/video voice/real-time 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 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, 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 VoIP, Real-time Text 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 Considerations......................................14
    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 VoIP,
    Real-time Text and Video over IP services relies 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 VoIP VoIP, Real-time 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 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
    VoIP, Real-time Text or Video over IP session.  A way to achieve
    this goal is to request an endpoint to loop media back to the other
    endpoint and to provide media statistics (e.g., 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 1*2(loopback-type-choice) [ space loopback-type-choice "rtp-start-
    loopback" ]
    loopback-type-choice = "rtp-pkt-loopback" | "rtp-media-loopback "rtp-media-loopback” |
    rtp-start-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 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.  The answering entity MAY
    include this attribute in the answer if it is not present in the
    offer.  This may be necessary if the answering entity is aware of
    NAT’s, firewalls, or RTP relays on the path of the call. In this
    case the offering entity MUST accept media according to
    rtp-start-loopback attribute.  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 receiver for
    transmission in the mirrored stream. stream unless rtp-start-loopback is
    requested

    The loopback mode attribute does not apply to rtp-start-loopback
    attribute and MUST be ignored if received by the answering entityt. entity.

 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 are 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 indicate 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.

    Note: NAT devices may change the actual port number that is used
    for transmission and the expected receive port.

 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 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 in the 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 entity that is compliant to this specification and
    accepting a media with rtp-pkt-loopback loopback-type MUST
    re-generate all of loopback
    the incoming RTP header fields as it does when
    transmitting other media.  However, the answering entity MUST
    maintain packets while re-generating only the timing information SSRC field of
    the received RTP packets when
    generating header.  Note that during the RTP timestamp for rtp-pkt-loopback mode the transmit packets.  Maintaining
    answering entity does not have control over the timing information encoding 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 cannot perform certain functions including congestion
    control on the answering entity and will not
    provide packet loss information to looped back media. However, since the receiver purpose of the
    loopback
    media. is to characterize the round-trip path at the RTP level,
    this limitation is acceptable.

    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 . 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 answering 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 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, and VoIP Metric Reports Block per sections
    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
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 224.2.17.12/127 192.168.0.12/127
    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 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
    is limited by the server.

 10. IANA Considerations

    There are no IANA considerations associated with this
    specification.

 11. Acknowledgements

    The authors wish to thank Nagarjuna Venna, 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: 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,
       Hughes Systique Corp.
       15245 Shady Grove Rd, Ste 330
       Rockville MD  20876 20850
       US

       Phone: +1 301 212 7860 527 1629
       EMail: arjun.roy@flextronicssoftware.com arjun@hsc.com
       URI:   http://www.flextronicssoftware.com/   http://www. hsc.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,

       663 Salem St.
       Lynnfield, MA  01851
       US 01940

       Phone: +1 410 908 7587
       EMail: nathan@robotics.net
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