





Network Working Group                                            R. Even
Request for Comments: 4587                                       Polycom
Obsoletes: 2032                                              August 2006
Category: Standards Track


               RTP Payload Format for H.261 Video Streams

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This memo describes a scheme to packetize an H.261 video stream for
   transport using the Real-time Transport Protocol, RTP, with any of
   the underlying protocols that carry RTP.

   The memo also describes the syntax and semantics of the Session
   Description Protocol (SDP) parameters needed to support the H.261
   video codec.  A media type registration is included for this payload
   format.

   This specification obsoletes RFC 2032.



















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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
   3. Structure of the Packet Stream ..................................3
      3.1. Overview of the ITU-T Recommendation H.261 .................3
      3.2. Considerations for Packetization ...........................4
   4. Specification of the Packetization Scheme .......................5
      4.1. Usage of RTP ...............................................5
      4.2. Recommendations for Operation with Hardware Codecs .........8
   5. Packet Loss Issues ..............................................9
   6. IANA Considerations ............................................10
      6.1. Media Type Registrations ..................................10
           6.1.1. Registration of MIME Media Type video/H261 .........10
      6.2. SDP Parameters ............................................12
           6.2.1. Usage with the SDP Offer Answer Model ..............12
   7. Backward Compatibility to RFC 2032 .............................13
      7.1. Optional H.261-Specific Control Packets ...................13
      7.2. New SDP Optional Parameters ...............................13
   8. Security Considerations ........................................14
   9. Acknowledgements ...............................................14
   10. Changes from RFC 2032 .........................................14
   11. References ....................................................15
      11.1. Normative References .....................................15
      11.2. Informative References ...................................15


























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1.  Introduction

   ITU-T Recommendation H.261 [H261] specifies the encoding used by
   ITU-T-compliant video-conference codecs.  Although this encoding was
   originally specified for fixed-data rate Integrated Services Digital
   Network (ISDN) circuits, experiments [INRIA], [MICE] have shown that
   they can also be used over packet-switched networks, such as the
   Internet.

   The purpose of this memo is to specify the RTP payload format for
   encapsulating H.261 video streams in RTP [RFC3550].

   This document obsoletes RFC 2032 and updates the "video/h261" media
   type that was registered in RFC 3555.

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 [RFC2119] and
   indicate requirement levels for compliant RTP implementations.

3.  Structure of the Packet Stream

3.1.  Overview of the ITU-T Recommendation H.261

   The H.261 coding is organized as a hierarchy of groupings.  The video
   stream is composed of a sequence of images, or frames, which are
   themselves organized as a set of Groups of Blocks (GOB).  Note that
   H.261 "pictures" are referred to as "frames" in this document.  Each
   GOB holds a set of 3 lines of 11 macro blocks (MB).  Each MB carries
   information on a group of 16x16 pixels: luminance information is
   specified for 4 blocks of 8x8 pixels, whereas chrominance information
   is given by two "red" and "blue" color difference components at a
   resolution of only 8x8 pixels.  These components and the codes
   representing their sampled values are as defined in ITU-R
   Recommendation 601 [BT601].

   This grouping is used to specify information at each level of the
   hierarchy:

   - At the frame level, one specifies information such as the delay
     from the previous frame, the image format, and various indicators.

   - At the GOB level, one specifies the GOB number and the default
     quantifier that will be used for the MBs.





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   - At the MB level, one specifies which blocks are present and which
     did not change, and, optionally, a quantifier and motion vectors.

   Blocks that have changed are encoded by computing the discrete cosine
   transform (DCT) of their coefficients, which are then quantized and
   Huffman encoded (Variable Length Codes).

   The H.261 Huffman encoding includes a special "GOB start" pattern,
   which is a word of 16 bits, 0000 0000 0000 0001.  This pattern is
   included at the beginning of each GOB header (and also at the
   beginning of each frame header) to mark the separation between two
   GOBs and is in fact used as an indicator that the current GOB is
   terminated.  The encoding also includes a stuffing pattern, composed
   of seven zero bits followed by four bits with a value of one; that
   stuffing pattern can only be entered between the encoding of MBs, or
   just before the GOB separator.

3.2.  Considerations for Packetization

   H.261 codecs designed for operation over ISDN circuits produce a bit
   stream composed of several levels of encoding specified by H.261 and
   companion recommendations.  The bits resulting from the Huffman
   encoding are arranged in 512-bit frames, containing 2 bits of
   synchronization, 492 bits of data and 18 bits of error correcting
   code.  The 512-bit frames are then interlaced with an audio stream
   and transmitted over px 64 kbps circuits according to specification
   H.221 [H221].

   For transmitting over the Internet, we will directly consider the
   output of the Huffman encoding.  All the bits produced by the Huffman
   encoding stage will be included in the packet.  We will not carry the
   512-bit frames, as protection against bit errors can be obtained by
   other means.  Similarly, we will not attempt to multiplex audio and
   video signals in the same packets, as UDP and RTP provide a much more
   suitable way to achieve multiplexing.

   Directly transmitting the result of the Huffman encoding over an
   unreliable stream of UDP datagrams would, however, have poor error
   resistance characteristics.  The result of the hierarchical structure
   of the H.261 bit stream is that one needs to receive the information
   present in the frame header to decode the GOBs, as well as the
   information present in the GOB header to decode the MBs.  Without
   precautions, this would mean that one has to receive all the packets
   that carry an image in order to decode its components properly.

   If each image could be carried in a single packet, this requirement
   would not create a problem.  However, a video image or even one GOB
   by itself can sometimes be too large to fit in a single packet.



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   Therefore, the MB is taken as the unit of fragmentation.  Packets
   must start and end on an MB boundary; that is, an MB cannot be split
   across multiple packets.  Multiple MBs may be carried in a single
   packet when they will fit within the maximal packet size allowed.
   This practice is recommended to reduce the packet send rate and
   packet overhead.

   To allow each packet to be processed independently for efficient
   resynchronization in the presence of packet losses, some state
   information from the frame header and GOB header is carried with each
   packet to allow the MBs in that packet to be decoded.  This state
   information includes the GOB number in effect at the start of the
   packet, the macroblock address predictor (i.e., the last macroblock
   address (MBA) encoded in the previous packet), the quantizer value in
   effect prior to the start of this packet (GQUANT, MQUANT, or zero in
   the case of a beginning of GOB) and the reference motion vector data
   (MVD) for computing the true MVDs contained within this packet.  The
   bit stream cannot be fragmented between a GOB header and MB 1 of that
   GOB.

   Moreover, since the compressed MB may not fill an integer number of
   octets, the data header contains two 3-bit integers, SBIT and EBIT,
   to indicate the number of unused bits in the first and last octets of
   the H.261 data, respectively.

4.  Specification of the Packetization Scheme

4.1.  Usage of RTP

   Each RTP packet starts with a fixed RTP header, as explained in RFC
   3550 [RFC3550].  The following fields of the RTP fixed header used
   for H.261 video streams are further emphasized here:

   - Payload type.  The assignment of an RTP payload type for this
     packet format is outside the scope of this document and will not be
     specified here.  It is expected that the RTP profile for a
     particular class of applications will assign a payload type for
     this encoding, or, if that is not done, then a payload type in the
     dynamic range shall be chosen.

   - The RTP timestamp encodes the sampling instant of the first video
     image contained in the RTP data packet.  If a video image occupies
     more than one packet, the timestamp SHALL be the same on all of
     those packets.  Packets from different video images MUST have a
     different timestamp so that frames may be distinguished by the
     timestamp.  For H.261 video streams, the RTP timestamp is based on
     a 90-kHz clock.  This clock rate is a multiple of the natural H.261
     frame rate (i.e., 30000/1001 or approximately 29.97 Hz).  That way,



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     for each frame time, the clock is just incremented by the multiple,
     and this removes inaccuracy in calculating the timestamp.
     Furthermore, the initial value of the timestamp MUST be random
     (unpredictable) to make known-plaintext attacks on encryption more
     difficult; see RTP [RFC3550].  Note that if multiple frames are
     encoded in a packet (e.g., when there are very few changes between
     two images), it is necessary to calculate display times for the
     frames after the first, using the timing information in the H.261
     frame header.  This is required because the RTP timestamp only
     gives the display time of the first frame in the packet.

   - The marker bit of the RTP header MUST be set to one in the last
     packet of a video frame; otherwise, it MUST be zero.  Thus, it is
     not necessary to wait for a following packet (which contains the
     start code that terminates the current frame) to detect that a new
     frame should be displayed.

   The H.261 data SHALL follow the RTP header, as in the following:

       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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       .                                                               .
       .                          RTP header                           .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          H.261  header                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          H.261 stream ...                     .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The H.261 header is defined as follows:

       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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |SBIT |EBIT |I|V| GOBN  |   MBAP  |  QUANT  |  HMVD   |  VMVD   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The fields in the H.261 header have the following meanings:

   Start bit position (SBIT): 3 bits

      Number of most significant bits that should be ignored in the
      first data octet.






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   End bit position (EBIT): 3 bits

      Number of least significant bits that should be ignored in the
      last data octet.

   INTRA-frame encoded data (I): 1 bit

      Set to 1 if this stream contains only INTRA-frame coded blocks.
      Set to 0 if this stream may or may not contain INTRA-frame coded
      blocks.  The meaning of this bit should not be changed during the
      course of the RTP session.

   Motion Vector flag (V): 1 bit

      Set to 0 if motion vectors are not used in this stream.  Set to 1
      if motion vectors may or may not be used in this stream.  The
      meaning of this bit should not be changed during the course of the
      session.

   GOB number (GOBN): 4 bits

      Encodes the GOB number in effect at the start of the packet.  Set
      to 0 if the packet begins with a GOB header.

   Macroblock address predictor (MBAP): 5 bits

      Encodes the macroblock address predictor (i.e., the last MBA
      encoded in the previous packet).  This predictor ranges from 0 -
      32 (to predict the valid MBAs 1 - 33), but because the bit stream
      cannot be fragmented between a GOB header and MB 1, the predictor
      at the start of the packet shall not be 0.  Therefore, the range
      is 1 - 32, which is biased by -1 to fit in 5 bits.  For example,
      if MBAP is 0, the value of the MBA predictor is 1.  Set to 0 if
      the packet begins with a GOB header.

   Quantizer (QUANT): 5 bits

      Quantizer value (MQUANT or GQUANT) in effect prior to the start of
      this packet.  Set to 0 if the packet begins with a GOB header.

   Horizontal motion vector data (HMVD): 5 bits

      Reference horizontal motion vector data (MVD).  Set to 0 if V flag
      is 0 or if the packet begins with a GOB header, or when the MTYPE
      of the last MB encoded in the previous packet was not motion
      compensation (MC).  HMVD is encoded as a 2s complement number, and
      '10000' corresponding to the value -16 is forbidden (motion vector
      fields range from +/-15).



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   Vertical motion vector data (VMVD): 5 bits

      Reference vertical motion vector data (MVD).  Set to 0 if V flag
      is 0 or if the packet begins with a GOB header, or when the MTYPE
      of the last MB encoded in the previous packet was not MC.  VMVD is
      encoded as a 2s complement number, and '10000' corresponding to
      the value -16 SHALL not be used (motion vector fields range from
      +/-15).

   Note that the I and V flags are hint flags; i.e., they can be
   inferred from the bit stream.  They are included to allow decoders to
   make optimizations that would not be possible if these hints were not
   provided before the bit stream was decoded.  Therefore, these bits
   cannot change for the duration of the stream.  A conforming
   implementation can always set V=1 and I=0.

   The H.261 stream SHALL be used without BCH error correction and
   without error correction framing.

4.2.  Recommendations for Operation with Hardware Codecs

   Packetizers for hardware codecs can trivially figure out GOB
   boundaries, using the GOB-start pattern included in the H.261 data.
   (Note that software encoders already know the boundaries.)  The
   cheapest packetization implementation is to packetize at the GOB
   level all the GOBs that fit in a packet.  But when a GOB is too
   large, the packetizer has to parse it to do MB fragmentation.  (Note
   that only the Huffman encoding must be parsed and that it is not
   necessary to decompress the stream fully, so this requires relatively
   little processing; examples of implementations can be found in some
   public H.261 codecs, such as IVS [IVS] and VIC [VIC].)  It is
   recommended that MB level fragmentation be used when feasible in
   order to obtain more efficient packetization.  Using this
   fragmentation scheme reduces the output packet rate and therefore
   reduces the overhead.

   At the receiver, the data stream can be depacketized and directed to
   a hardware codec's input.  If the hardware decoder operates at a
   fixed bit rate, synchronization may be maintained by inserting the
   stuffing pattern between MBs (i.e., between packets) when the packet
   arrival rate is slower than the bit rate.










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5.  Packet Loss Issues

   On the Internet, most packet losses are due to network congestion
   rather than to transmission errors.  Using UDP, no mechanism is
   available at the sender to know whether a packet has been
   successfully received.  It is up to the application (i.e., coder and
   decoder) to handle the packet loss.  Each RTP packet includes a
   sequence number field that can be used to detect packet loss.

   H.261 uses the temporal redundancy of video to perform compression.
   This differential coding (or INTER-frame coding) is sensitive to
   packet loss.  After a packet loss, parts of the image may remain
   corrupt until all corresponding MBs have been encoded in INTRA-frame
   mode (i.e., encoded independently of past frames).  There are several
   ways to mitigate packet loss:

   (1)  One way is to use only INTRA-frame encoding and MB-level
        conditional replenishment.  That is, only MBs that change
        (beyond some threshold) are transmitted.

   (2)  Another way is to adjust the INTRA-frame encoding refreshment
        rate according to the packet loss observed by the receivers.
        The H.261 recommendation specifies that an MB be INTRA-frame
        encoded at least every 132 times it is transmitted.  However,
        the INTRA-frame refreshment rate can be raised in order to speed
        the recovery when the measured loss rate is significant.

   (3)  The fastest way to repair a corrupted image is to request an
        INTRA-frame coded image refreshment after a packet loss is
        detected.  One means to accomplish this is for the decoder to
        send to the coder a list of packets lost.  The coder can decide
        to encode every MB of every GOB of the following video frame in
        INTRA-frame mode (i.e., full INTRA-frame encoded).  If the coder
        can deduce from the packet sequence numbers which MBs were
        affected by the loss, it can save bandwidth by sending only
        those MBs in INTRA-frame mode.  This mode is particularly
        efficient in point-to-point connection or when the number of
        decoders is low.

   The H.261-specific control packets FIR and NACK, as described in RFC
   2032, SHALL NOT be used to request image refreshment.  Old
   implementations are encouraged to use the methods described in this
   section.  Image refreshment may be needed due to packet loss or due
   to application requirements.  An example of application requirement
   may be the change of the speaker in a voice-activated multipoint
   video switching conference.  There are two methods that can be used
   for requesting image refreshment.  The first method is by using the
   Extended RTP Profile for RTCP-based Feedback and sending RTCP generic



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   control packets, as described in RFC 4585 [RFC4585].  The second
   method is by using application protocol-specific commands, such as
   H.245 [ITU.H245] FastUpdateRequest.

6.  IANA Considerations

   This section updates the H.261 media type described in RFC 3555
   [RFC3555].

   This section specifies optional parameters that MAY be used to select
   optional features of the payload format.  The parameters are
   specified here as part of the MIME subtype registration for the ITU-T
   H.261 codec.  A mapping of the parameters into the Session
   Description Protocol (SDP) [RFC4566] is also provided for those
   applications that use SDP.  Multiple parameters SHOULD be expressed
   as a media type string, in the form of a semicolon-separated list of
   parameters.

6.1.  Media Type Registrations

   This section describes the media types and names associated with this
   payload format.  The section updates the previous registered version
   in RFC 3555 [RFC3555].  This registration uses the template defined
   in RFC 4288 [RFC4288]

6.1.1.  Registration of MIME Media Type video/H261

   MIME media type name: video

   MIME subtype name: H261

   Required parameters: None

   Optional parameters:

      CIF.  This parameter has the format of parameter=value.  It
      describes the maximum supported frame rate for CIF resolution.
      Permissible values are integer values 1 to 4, and it means that
      the maximum rate is 29.97/specified value.

      QCIF.  This parameter has the format of parameter=value.  It
      describes the maximum supported frame rate for QCIF resolution.
      Permissible values are integer values 1 to 4, and it means that
      the maximum rate is 29.97/specified value.







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      D.  Specifies support for still image graphics according to H.261,
      annex D.  If supported, the parameter value SHALL be "1".  If not
      supported, the parameter SHOULD NOT be used or SHALL have the
      value "0".

   Encoding considerations:

      This media type is framed and binary, see Section 4.8 in
      [RFC4288].

   Security considerations: See Section 8

   Interoperability considerations:

      These are receiver options; current implementations will not send
      any optional parameters in their SDP.  They will ignore the
      optional parameters and will encode the H.261 stream without annex
      D.  Most decoders support at least QCIF resolutions, and they are
      expected to be available in almost every H.261-based video
      application.

   Published specification: RFC 4587

   Applications that use this media type:

      Audio and video streaming and conferencing applications.

   Additional information: None

   Person and email address to contact for further information:

      Roni Even: roni.even@polycom.co.il

   Intended usage: COMMON

   Restrictions on usage:

      This media type depends on RTP framing and thus is only defined
      for transfer via RTP [RFC3550].  Transport within other framing
      protocols is not defined at this time.

   Author: Roni Even

   Change controller:

      IETF Audio/Video Transport working group, delegated from the IESG.





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6.2.  SDP Parameters

   The MIME media type video/H261 string is mapped to fields in the
   Session Description Protocol (SDP) as follows:

   o  The media name in the "m=" line of SDP MUST be video.

   o  The encoding name in the "a=rtpmap" line of SDP MUST be H261 (the
      MIME subtype).

   o  The clock rate in the "a=rtpmap" line MUST be 90000.

   o  The optional parameters "CIF", "QCIF", and "D", if any, SHALL be
      included in the "a=fmtp" line of SDP.  These parameters are
      expressed as a MIME media type string, in the form of as a
      semicolon-separated list of parameters

6.2.1.  Usage with the SDP Offer Answer Model

   When H.261 is offered over RTP using SDP in an Offer/Answer model
   [RFC3264] the following considerations are necessary.

   Codec options: (D) This option MUST NOT appear unless the sender of
   this SDP message is able to decode this option.  This option SHALL be
   considered a receiver's capability even when it is sent in a
   "sendonly" offer.

   Picture sizes and MPI:

   Supported picture sizes and their corresponding minimum picture
   interval (MPI) information for H.261 can be combined.  All picture
   sizes may be advertised to the other party, or only a subset of it.
   Using the recvonly or sendrev direction attribute, a terminal SHOULD
   announce those picture sizes (with their MPIs) that it is willing to
   receive.  For example, CIF=2 means that receiver can receive a CIF
   picture and that the frame rate SHALL be less then 15 frames per
   second.

   When the direction attribute is sendonly, the parameters describe the
   capabilities of the stream that the sender can produce.

   Implementations following this specification SHALL specify at least
   one supported picture size.

   If the receiver does not specify the picture size/MPI parameter, then
   it is safe to assume that it is an implementation that follows RFC
   2032.  In that case, it is RECOMMENDED to assume that such a receiver
   is able to support reception of QCIF resolution with MPI=1.



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   Parameters offered first are the most preferred picture mode to be
   received.

   An example of media representation in SDP is as follows CIF at 15
   frames per second, QCIF at 30 frames per second and annex D

      m=video 49170/2 RTP/AVP 31
      a=rtpmap:31 H261/90000
      a=fmtp:31 CIF=2;QCIF=1;D=1

   This means that the sender of this message can decode an H.261 bit
   stream with the following options and parameters: preferred
   resolution is CIF (its MPI is 2), but if that is not possible, then
   QCIF size is also supported.  Still image using annex D MAY be used.

7.  Backward Compatibility to RFC 2032

   The current document replaces RFC 2032.  This section will address
   the major backward compatibility issues.

7.1.  Optional H.261-Specific Control Packets

   RFC 2032 defined two H.261-specific RTCP control packets, "Full
   INTRA-frame Request" and "Negative Acknowledgement".  Support of
   these control packets was optional.  The H.261-specific control
   packets differ from normal RTCP packets in that they are not
   transmitted to the normal RTCP destination transport address for the
   RTP session (which is often a multicast address).  Instead, these
   control packets are sent directly via unicast from the decoder to the
   encoder.  The destination port for these control packets is the same
   port that the encoder uses as a source port for transmitting RTP
   (data) packets.  Therefore, these packets may be considered "reverse"
   control packets.  This memo suggests generic methods to address the
   same requirement.  The authors of the documents are not aware of
   products that support these control packets.  Since these are
   optional features, new implementations SHALL ignore them, and they
   SHALL NOT be used by new implementations.

7.2.  New SDP Optional Parameters

   The document adds new optional parameters to the H261 payload type.
   Since these are optional parameters, we expect that old
   implementations ignore these parameters, whereas new implementations
   that receive the H261 payload type capabilities with no parameters
   will assume that it is an old implementation and will send H.261 at
   QCIF resolution and 30 frames per second.





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8.  Security Considerations

   RTP packets using the payload format defined in this specification
   are subject to the security considerations discussed in the RTP
   specification [RFC3550], and in any appropriate RTP profile (e.g.,
   [RFC3551]).  This implies that confidentiality of the media streams
   is achieved by encryption.  SRTP [RFC3711] may be used to provide
   both encryption and integrity protection of RTP flow.  Because the
   data compression used with this payload format is applied end to end,
   encryption will be performed after compression, so there is no
   conflict between the two operations.

   A potential denial-of-service threat exists for data encoding using
   compression techniques that have non-uniform receiver-end
   computational load.  The attacker can inject pathological datagrams
   into the stream that are complex to decode and cause the receiver to
   be overloaded.  The usage of authentication of at least the RTP
   packet is RECOMMENDED.  H.261 is vulnerable to such attacks because
   it is possible for an attacker to generate RTP packets containing
   frames that affect the decoding process of future frames.  Therefore,
   the usage of data origin authentication and data integrity protection
   of at least the RTP packet is RECOMMENDED; for example, with SRTP.

   Note that the appropriate mechanism to ensure confidentiality and
   integrity of RTP packets and their payloads is very dependent on the
   application and on the transport and signaling protocols employed.
   Thus, although SRTP is given as an example above, other possible
   choices exist.

9.  Acknowledgements

   This is to acknowledge the authors of RFC 2032, Thierry Turletti and
   Christian Huitema.  Special thanks for the work done by Petri
   Koskelainen from Nokia and Nermeen Ismail from Cisco, who helped with
   drafting the text for the new MIME types.

10.  Changes from RFC 2032

   The changes from the RFC 2032 are:

   1.  The H.261 MIME type is now in the payload specification.

   2.  Added optional parameters to the H.261 MIME type

   3.  Deprecated the H.261 specific control packets

   4.  Editorial changes to be in line with RFC editing procedures




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11.  References

11.1.  Normative References

   [H261]      International Telecommunications Union, "Video codec for
               audiovisual services at px 64 kbit/s", ITU Recommendation
               H.261, March 1993.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]   Schulzrinne, H., Casner, S., Frederick, R., and V.
               Jacobson, "RTP: A Transport Protocol for Real-Time
               Applications", STD 64, RFC 3550, July 2003.

   [RFC3264]   Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
               with Session Description Protocol (SDP)", RFC 3264,
               June 2002.

   [RFC3551]   Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
               Video Conferences with Minimal Control", STD 65,
               RFC 3551, July 2003.

   [RFC3555]   Casner, S. and P. Hoschka, "MIME Type Registration of RTP
               Payload Formats", RFC 3555, July 2003.

   [RFC4566]   Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
               Description Protocol", RFC 4566, July 2006.

11.2.  Informative References

   [RFC4288]   Freed, N. and J. Klensin, "Media Type Specifications and
               Registration Procedures", BCP 13, RFC 4288,
               December 2005.

   [RFC4585]   Ott, J., Wenger, S., Sato, N., Burmeister, C., and J.
               Rey, "Extended RTP Profile for Real-time Transport
               Control Protocol (RTCP)-based Feedback (RTP/AVPF)", RFC
               4585, July 2006.

   [ITU.H245]  International Telecommunications Union, "CONTROL PROTOCOL
               FOR MULTIMEDIA COMMUNICATION", ITU Recommendation H.245,
               2003.

   [INRIA]     Turletti, T., "H.261 software codec for videoconferencing
               over the Internet", INRIA Research Report 1834,
               January 1993.




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   [IVS]       Turletti, T., "INRIA Videoconferencing tool (IVS)",
               available by anonymous ftp from zenon.inria.fr in the
               "rodeo/ivs/last_version" directory.  See also URL
               <http://www.inria.fr/rodeo/ivs.html>.

   [BT601]     International Telecommunications Union, "Studio encoding
               parameters of digital television for standard 4:3 and
               wide-screen 16:9 aspect ratios", ITU-R Recommendation
               BT.601-5, October 1995.

   [MICE]      Sasse, MA., Bilting, U., Schultz, CD., and T.  Turletti,
               "Remote Seminars through MultiMedia Conferencing:
               Experiences from the MICE project", Proc. INET'94/JENC5,
               Prague pp. 251/1-251/8, June 1994.

   [VIC]       MacCanne, S., "VIC Videoconferencing tool, available by
               anonymous ftp from ee.lbl.gov in the "conferencing/vic"
               directory".

   [RFC3711]   Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
               Norrman, "The Secure Real-time Transport Protocol
               (SRTP)", RFC 3711, March 2004.

   [H221]      International Telecommunications Union, "Frame structure
               for a 64 to 1920 kbit/s channel in audiovisual
               teleservices", ITU Recommendation H.221, May 1999.

Author's Address

   Roni Even
   Polycom
   94 Derech Em Hamoshavot
   Petach Tikva  49130
   Israel

   EMail: roni.even@polycom.co.il















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