[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 RFC 6849

                                                             K. Hedayat
  Internet Draft                                          Brix Networks
  Expires: June 2008                                           P. Jones
                                                    Cisco Systems, Inc.
                                                        A. Roychowdhury
                                                                 Hughes
                                                         C. SivaChelvan
                                                    Cisco Systems, Inc.
                                                            N. Stratton
 
                                                          November 2007
 
     An Extension to the Session Description Protocol (SDP) for Media
                                Loopback
                   draft-ietf-mmusic-media-loopback-07
 
 
 Status of this Memo
 
 
    By submitting this Internet-Draft, each author represents that any
    applicable patent or other IPR claims of which he or she is aware
    have been or will be disclosed, and any of which he or she becomes
    aware will be disclosed, in accordance with Section 6 of BCP 79.
 
    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note that
    other groups may also distribute working documents as
    Internet-Drafts.
 
    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other
    documents at any time.  It is inappropriate to use Internet-Drafts
    as reference material or to cite them other than as "work in
    progress."
 
    The list of current Internet-Drafts can be accessed at
         http://www.ietf.org/ietf/1id-abstracts.txt
    The list of Internet-Draft Shadow Directories can be accessed at
         http://www.ietf.org/shadow.html.
 
 
 Copyright Notice
 
    Copyright (C) The IETF Trust (2007).
 
 
 Hedayat, et al.           Expires June 2008                  [Page 1]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
 
 Abstract
 
    The wide deployment of Voice over IP (VoIP), Real-time Text and
    Video over IP services has introduced new challenges in managing
    and maintaining 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,
    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, Real-time Text and
    Video Over IP performance metrics.
 
 
 Table of Contents
 
 
    1. Introduction..................................................3
    2. Terminology...................................................4
    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.................7
       5.4 Generating the Answer for Loopback Session................8
       5.5 Offerer Processing of the Answer..........................9
       5.6 Modifying the Session....................................10
    6. RTP Requirements.............................................10
    7. Payload formats for Packet loopback..........................10
       7.1 Encapsulated Payload format..............................11
       7.2 Direct loopback RTP payload format.......................13
    8. RTCP Requirements............................................14
    9. Congestion Control...........................................15
    10. Examples....................................................15
       10.1 Offer for specific media loopback type..................15
       10.2 Offer for choice of media loopback type.................16
       10.3 Offer for choice of media loopback type with
       rtp-start-loopback...........................................17
 
 Hedayat, et al.           Expires June 2008                  [Page 2]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
       10.4 Response to INVITE request rejecting loopback media.....18
       10.5 Response to INVITE request rejecting loopback media with
       rtp-start-loopback...........................................18
    11. Security Considerations.....................................19
    12. Implementation Considerations...............................20
    13. IANA Considerations.........................................20
    14. Acknowledgements............................................20
    15. Normative References........................................20
 
 
 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 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, 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
    [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.
 
 
 
 
 
 
 
 
 Hedayat, et al.           Expires June 2008                  [Page 3]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
 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.
 
 
 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
    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:
 
 Hedayat, et al.           Expires June 2008                  [Page 4]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
 
       a=loopback:<loopback-type>
 
    Following is the Augmented BNF [RFC2234] for loopback-type:
 
    loopback-type = loopback-type-choices | loopback-type-choice-3
    loopback-choices = loopback-type-choice-1 | loopback-type-choice-2
    | loopback-type-choice-1 1*space loopback-type-choice-2 |
    loopback-type-choice-2 1*space loopback-type-choice-1
    loopback-type-choice-1 = "rtp-pkt-loopback"
    loopback-type-choice-2 = "rtp-media-loopback"
    loopback-type-choice-3 = "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. 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.
 
    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 loopback-source is blocked by a network
    element until the loopback-mirror starts transmitting packets.
    Loopback-source and loopback-mirror are loopback modes defined in
    section 5.2.  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 loopback-source's packets are dropped by the
    RTP relay until the loopback-mirror 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 loopback-mirror prior to the
    loopback process for the purpose of setting media state within the
    network.  In the presence of this loopback attribute the loopback-
    mirror will transmit media, according to the description that
    contains this attribute, until it receives media from the loopback-
 
 Hedayat, et al.           Expires June 2008                  [Page 5]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    source.  The loopback-mirror MAY include this attribute in the
    answer if it is not present in the offer.  This may be necessary if
    the loopback-mirror is aware of NAT's, firewalls, or RTP relays on
    the path of the call. In this case the loopback-source MUST accept
    media according to rtp-start-loopback attribute.  After the first
    media packet is received from the loopback-source, the loopback-
    mirror 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 loopback-source 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 are additional
    mode attributes like sendonly, recvonly, etc.  The syntax of the
    loopback mode media attribute is:
 
       a=<loopback-mode>:<fmt>...
 
    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 receiver for
    transmission in the mirrored stream unless rtp-start-loopback is
    requested.
 
    <fmt> is a media format description. The format descrption has the
    semantics as defined in section 5.14 of RFC 4566 [RFC2234]. When
    loopback-mode is specified as loopback-source, the media format
    corresponds to the RTP payload types the source is willing to send.
 
 Hedayat, et al.           Expires June 2008                  [Page 6]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    When loopback-mode is specified as loopback-mirror, the media
    format corresponds to the RTP payload types the mirror is willing
    to receive. The payload types specified in m= line for a
    loopback-source specify the payloads the source is willing to
    receive. Similarly, for the loopback-mirror these payload types
    specify the payloads it is willing to send.
 
    The loopback mode attribute does not apply to rtp-start-loopback
    attribute and MUST be ignored if received by the answering 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:   m=audio 41352 RTP/AVP 0 8
               a=loopback:rtp-media-loopback
               a=loopback-source:0 8
 
    Note: A loopback offer in a given media description MUST NOT
    contain the standard mode attributes sendonly, recvonly, sendrecv
    or inactive. The loopback-mode attributes (loopback-source and
    loopback-mirror) replace the standard attributes.
 
    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 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 (m= line) 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.
    The RTP payload types indicated in the a=loopback-source line are
    the payload types for the codecs the offerer is willing 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.
 
    If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send
    and the loopback-source MUST receive the looped back packets
    encoded in one of the two payload formats (encapsulated RTP or
    payload loopback) as defined in section 7.
 
    Example:   m=audio 41352 RTP/AVP 112
               a=loopback:rtp-pkt-loopback
 
 Hedayat, et al.           Expires June 2008                  [Page 7]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
               a=loopback-source:0 8
               a=rtpmap:112 encaprtp/8000
 
    Example:   m=audio 41352 RTP/AVP 112
               a=loopback:rtp-pkt-loopback
               a=loopback-source:0 8
               a=rtpmap:112 rtploopback/8000
 
    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:
 
       m=audio 41352 RTP/AVP 0 8
       a=loopback:rtp-media-loopback
       a=loopback-source:0 8
 
    The answer that is capable of supporting the offer MUST contain:
 
       m=audio 41352 RTP/AVP 0 8
       a=loopback:rtp-media-loopback
       a=loopback-mirror:0 8
 
    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:
 
       m=audio 41352 RTP/AVP 0 8 112
       a=loopback:rtp-media-loopback rtp-pkt-loopback
       a=loopback-source:0 8
 
    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 41352 RTP/AVP 0 8
 
 Hedayat, et al.           Expires June 2008                  [Page 8]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
       a=loopback:rtp-media-loopback
       a=loopback-mirror:0 8
 
    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 112 113
       a=loopback:rtp-pkt-loopback
       a=loopback-source:0 8
       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 41352 RTP/AVP 112
       a=loopback:rtp-pkt-loopback
       a=loopback-mirror:0 8
       a=rtpmap:112 encaprtp/8000
 
       m=audio 41352 RTP/AVP 113
       a=loopback:rtp-pkt-loopback
       a=loopback-mirror:0 8
       a=rtpmap:113 rtploopback/8000
 
 
 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.  The loopback-
    mode line MUST contain at least one codec the answerer is willing
    to send or receive depending on whether it is the loopback-source
    or the loopback-mirror. In addition, the "m=" line MUST contain at
    least one codec that the answerer is willing to send or receive
 
 Hedayat, et al.           Expires June 2008                  [Page 9]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    depending on whether it is the loopback-mirror or the loopback-
    source.
    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 entity that is compliant to this specification and
    accepting a media with rtp-pkt-loopback loopback-type MUST loopback
    the incoming RTP packets using either the encapsulated RTP payload
    format or the direct loopback RTP payload format as defined in
    section 7 of this specification.
 
    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 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. 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
 
 Hedayat, et al.           Expires June 2008                 [Page 10]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    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 requirement 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 different packet, the encapsulated packet
    MAY be fragmented only if required (for example: due to MTU
    limitations).
 
 
 7.1.1 Usage of RTP Header fields
 
 
    Payload Type (PT): The assignment of an RTP payload type for this
    packet format is outside the scope of this document; it is either
    specified by the RTP profile under which this payload format is
    used or more likely signaled dynamically out-of-band (e.g., using
    SDP; section 7.3 defines the name binding).
 
    Marker (M) bit: If the received RTP packet is looped back in
    multiple RTP packets, the M bit is set to 1 in 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.
 
    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 based on the same clock 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]).
 
    SSRC: set as described in RFC 3550 [RFC3550].
 
    CC and CSRC fields are used as described in RFC 3550 [RFC3550].
 
 
 
 Hedayat, et al.           Expires June 2008                 [Page 11]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
 7.1.2 RTP Payload Structure
 
 
    The RTP header in the encapsulated packet MUST be followed by the
    payload header defined in this section. If the received RTP packet
    has to be looped back in multiple packets due to fragmentation, the
    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 usefully decode looped back packets in the
    presence of moderate packet loss [RFC3550].
 
     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 RTP
    header in the received packet except for the first 4 bits of the
    first octet.
 
    Receive Timestamp: 32 bits
 
    The Receieve timestamp denotes the sampling instant for when the
    last octet of the media packet that is being encapsulated by the
    loopback-mirror is received from the loopback-source.  The Receive
    timestamp MUST be based on the same clock 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.
 
 
 Hedayat, et al.           Expires June 2008                 [Page 12]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    Reserved: 2 bits
 
    This field is reserved for future definition. In the absence of
    such a definition, the bits in this field MUST be set to zero and
    MUST be ignored by the receiver.
 
    Any padding octets in the original packet MUST not be included in
    the loopback packet generated by a loopback-mirror. The
    loopback-mirror MAY add padding octets if required.
 
 
 7.1.3 Usage of SDP
 
 
    The payload type number for the encapsulated stream can be
    negotiated using a mechanism like SDP. There is no static payload
    type assignment for the encapsulated stream, so dynamic payload
    type numbers MUST be used.  The binding to the name is indicated by
    an rtpmap attribute.  The name used in this binding is "encaprtp".
 
    The following is an example SDP fragment for encapsulated RTP.
 
    m=audio 41352 RTP/AVP 112
    a=rtpmap:112 encaprtp/8000
 
 
 7.2 Direct loopback RTP payload format
 
 
    The direct loopback RTP payload format can be used in scenarios
    where the 16 byte overhead of the encapsulated payload format is
    significant.  This payload format MUST not be used in cases where
    the MTU on the loopback path is less than the MTU on the transmit
    path.  When using this payload format, the receiver MUST loop back
    each received packet in a separate RTP packet.
 
 
 7.2.1 Usage of RTP Header fields
 
 
    Payload Type (PT): The assignment of an RTP payload type for this
    packet format is outside the scope of this document; it is either
    specified by the RTP profile under which this payload format is
    used or more likely signaled dynamically out-of-band (e.g., using
    SDP; section 7.3 defines the name binding).
 
    Marker (M) bit: Set to the value in the received packet.
 
    Extension (X) bit: Defined by the RTP Profile used.
 
 Hedayat, et al.           Expires June 2008                 [Page 13]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
 
    Sequence Number: The RTP sequence number SHOULD be generated by the
    loopback-mirror in the usual manner with a constant random offset.
 
    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 based on the same clock
    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]).
 
    SSRC: set as described in RFC 3550 [RFC3550].
 
    CC and CSRC fields are used as described in RFC 3550 [RFC3550].
 
 
 
 7.2.2 RTP Payload Structure
 
 
    This payload format does not define any payload specific headers.
    The loopback-mirror simply copies the payload data from the payload
    portion of the 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 encapsulated RTP.
 
    m=audio 41352 RTP/AVP 112
    a=rtpmap:112 rtploopback/8000
 
 8. 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
 
 Hedayat, et al.           Expires June 2008                 [Page 14]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    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].
 
 9. Congestion Control
 
 
    All the participants in a 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.
 
 
 10. 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.
 
 
 10.1 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 192.0.2.11
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=loopback:rtp-media-loopback
    a=loopback-source:0
 
    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 192.0.2.11
    s=Example
 
 Hedayat, et al.           Expires June 2008                 [Page 15]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=loopback:rtp-media-loopback
    a=loopback-mirror:0
 
    The server is accepting to mirror the media from the client at the
    media level.
 
 
 10.2 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 192.0.2.11
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 49170 RTP/AVP 0 112 113
    a=loopback:rtp-media-loopback rtp-pkt-loopback
    a=loopback-source:0
    a=rtpmap:112 encaprtp/8000
    a=rtpmap:113 rtploopback/8000
 
 
    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 192.0.2.11
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 49170 RTP/AVP 112
    a=loopback:rtp-pkt-loopback
    a=loopback-mirror:0
    a=rtpmap:112 encaprtp/8000
 
 
 
 Hedayat, et al.           Expires June 2008                 [Page 16]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    The server is accepting to mirror the media from the client at the
    packet level using the encapsulated RTP payload format.
 
 
 10.3 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 192.0.2.11
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 49170 RTP/AVP 0 112 113
    a=loopback:rtp-media-loopback rtp-pkt-loopback
    a=loopback-source:0
    a=rtpmap:112 encaprtp/8000
    a=rtpmap:113 rtploopback/8000
    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 192.0.2.11
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 49170 RTP/AVP 113
    a=loopback:rtp-pkt-loopback
    a=loopback-mirror:0
    a=rtpmap:113 rtploopback/8000
    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 using the direct loopback RTP payload format.  The
 
 Hedayat, et al.           Expires June 2008                 [Page 17]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    server is also accepting to source media until it receives media
    packets from the client.
 
 
 10.4 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 192.0.2.11
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=loopback:rtp-media-loopback
    a=loopback-source:0
 
    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 192.0.2.11
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 0 RTP/AVP 0
    a=loopback:rtp-media-loopback
    a=loopback-mirror:0
 
    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.
 
 
 10.5 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 192.0.2.11
 
 Hedayat, et al.           Expires June 2008                 [Page 18]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 49170 RTP/AVP 0
    a=loopback:rtp-media-loopback
    a=loopback-source:0
    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 192.0.2.11
    s=Example
    i=An example session
    e=user@example.com
    c=IN IP4 192.0.2.12/127
    t=0 0
    m=audio 0 RTP/AVP 0
    a=loopback:rtp-media-loopback
    a=loopback-mirror:0
    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.
 
 
 
 11. 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
    is limited by the server.
 
 
 
 Hedayat, et al.           Expires June 2008                 [Page 19]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
 12. Implementation Considerations
 
    The media loopback approach described in this document is a
    complete solution that would work under all scenarios. However, it
    is believed that the solution may not be light-weight enough for
    the common case. 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 support the rtp-pkt-loopback option for
    loopback-type attribute. In addition, for the loopback-mode
    attribute, all implementations of an offerer MUST at a minimum be
    able to act as a loopback-source. All implementation MUST also at a
    minimum support the direct media loopback payload type. Remaining
    attribute values including rtp-media-loopback and
    rtp-start-loopback MAY be implemented in complete implementations
    of this draft.
 
 
 13. IANA Considerations
 
 
    There are no IANA considerations associated with this
    specification.
 
 
 14. Acknowledgements
 
 
    The authors wish to thank Nagarjuna Venna, Flemming Andreasen, Jeff
    Bernstein, Paul Kyzivat, and Dave Oran for their comments and
    suggestions.
 
 
 15. 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, June 2002.
 
 
       [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
                  Model with the Session Description Protocol (SDP)",
                  RFC 3264, June 2002.
 
 
 
 Hedayat, et al.           Expires June 2008                 [Page 20]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
       [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.
 
 
       [RFC2234] Crocker, P. Overell, "Augmented ABNF for Syntax
                  Specification: ABNF", RFC 2234, November 1997.
 
       [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.
 
 
 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
 
 Hedayat, et al.           Expires June 2008                 [Page 21]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
       US
 
       Phone: +1 919 392 6948
       EMail: paulej@packetizer.com
       URI:   http://www.cisco.com/
 
 
       Arjun Roychowdhury
       Hughes Systique Corp.
       15245 Shady Grove Rd, Ste 330
       Rockville MD 20850
       US
 
       Phone: +1 301 527 1629
       EMail: arjun@hsc.com
       URI:   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
 
       663 Salem St.
       Lynnfield, MA 01940
 
 
       Phone: +1 410 908 7587
       EMail: nathan@robotics.net
       URI:   http://www.robotics.net/
 
 
 Full Copyright Statement
 
    Copyright (C) The IETF Trust (2007).
 
 
    This document is subject to the rights, licenses and restrictions
    contained in BCP 78, and except as set forth therein, the authors
    retain all their rights.
 
 
 Hedayat, et al.           Expires June 2008                 [Page 22]
 

 Internet-Draft            SDP Media Loopback             November 2007
 
    This document and the information contained herein are provided on
    an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
    REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE
    IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL
    WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY
    WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE
    ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
    FOR A PARTICULAR PURPOSE.
 
 
 Intellectual Property
 
    The IETF takes no position regarding the validity or scope of any
    Intellectual Property Rights or other rights that might be claimed
    to pertain to the implementation or use of the technology described
    in this document or the extent to which any license under such
    rights might or might not be available; nor does it represent that
    it has made any independent effort to identify any such rights.
    Information on the procedures with respect to rights in RFC
    documents can be found in BCP 78 and BCP 79.
 
    Copies of IPR disclosures made to the IETF Secretariat and any
    assurances of licenses to be made available, or the result of an
    attempt made to obtain a general license or permission for the use
    of such proprietary rights by implementers or users of this
    specification can be obtained from the IETF on-line IPR repository
    at http://www.ietf.org/ipr.
 
    The IETF invites any interested party to bring to its attention any
    copyrights, patents or patent applications, or other proprietary
    rights that may cover technology that may be required to implement
    this standard.  Please address the information to the IETF at
    ietf-ipr@ietf.org.
 
    Acknowledgement
 
    Funding for the RFC Editor function is provided by the IETF
    Administrative Support Activity (IASA).
 
 
 
 
 
 
 
 
 
 
 
 
 Hedayat, et al.           Expires June 2008                 [Page 23]
 

Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/