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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 4587

AVT                                                              R. Even
Internet-Draft                                                   Polycom
Expires: August 17, 2005                               February 16, 2005


               RTP Payload Format for H.261 Video Streams
                   draft-ietf-avt-rfc2032-bis-05.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  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 become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at
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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 17, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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.

   This specification is a product of the Audio/Video Transport working
   group within the Internet Engineering Task Force.  Comments are
   solicited and should be addressed to the working group's mailing list
   and/or the authors.



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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Structure of the packet stream . . . . . . . . . . . . . . . .  5
     3.1   Overview of the ITU-T recommendation H.261 . . . . . . . .  5
     3.2   Considerations for packetization . . . . . . . . . . . . .  5
   4.  Specification of the packetization scheme  . . . . . . . . . .  7
     4.1   Usage of RTP . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2   Recommendations for operation with hardware codecs . . . .  9
   5.  Packet loss issues . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Payload Format Parameters  . . . . . . . . . . . . . . . . . . 13
     6.1   IANA Considerations  . . . . . . . . . . . . . . . . . . . 13
       6.1.1   Registration of MIME media type video/H261 . . . . . . 13
     6.2   SDP Parameters . . . . . . . . . . . . . . . . . . . . . . 14
       6.2.1   Usage with the SDP Offer Answer Model  . . . . . . . . 14
   7.  Backward Compatibility to RFC2032  . . . . . . . . . . . . . . 16
     7.1   Optional H.261-specific control packets  . . . . . . . . . 16
     7.2   New SDP optional parameters  . . . . . . . . . . . . . . . 16
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   10.   Changes from previous versions . . . . . . . . . . . . . . . 19
     10.1  changes from RFC 2032> . . . . . . . . . . . . . . . . . . 19
     10.2  Changes from bis-00  . . . . . . . . . . . . . . . . . . . 19
     10.3  Changes from bis-04  . . . . . . . . . . . . . . . . . . . 19
   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 20
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 20
   11.2  Informative References . . . . . . . . . . . . . . . . . . . 20
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 21
       Intellectual Property and Copyright Statements . . . . . . . . 22





















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1.  Introduction

   The ITU-T recommendation H.261[H261] specifies the encoding used by
   ITU-T compliant video-conference codecs.  Although these encoding
   were originally specified for fixed data rate 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 updates RFC2032.  The intention is to move the new RFC
   to draft standard.






































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













































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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 as "frames" in this document.  Each GOB
   holds a set of 3 lines of 11 macro blocks (MB).  each MB of 16x16
   pixels the luminance information is encoded for each pixel, sub
   divided into four 8x8 groups.  While the chrominance information is
   subsampled, for 2x2 pixels, resulting in color resolution of 8x8
   within the MB.  These components and the codes representing their
   sampled values are as defined in the 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.

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

   Blocks which 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



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   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 px64 kbps circuits according to specification
   H.221 [H221].

   When 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
   efficient 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 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 properly decode its components.

   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.
   Therefore, the MB is taken as the unit of fragmentation.  Packets
   must start and end on a MB boundary, i.e.  a 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 MBA encoded
   in the previous packet), the quantizer value in effect prior to the
   start of this packet (GQUANT, MQUANT or zero in 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 three-bit integers, SBIT and
   EBIT, to indicate the number of unused bits in the first and last
   octets of the H.261 data, respectively.



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4.  Specification of the packetization scheme

4.1  Usage of RTP

   The H.261 information is carried as payload data within the RTP
   protocol.  The following fields of the RTP header are specified:

   -    Payload type: According to RFC 3551[RFC3551] table 5,  Static
   payload type 31 SHALL be used to specify the H.261 video payload
   format for RTP/AVP.

   -    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
   different timestamp so that frames may be distinguished by the
   timestamp.  For H.261 video streams, the RTP timestamp is based on a
   90kHz clock.  This clock rate is a multiple of the natural H.261
   frame rate (i.e.  30000/1001 or approx.  29.97 Hz).  That way, 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 little 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, and otherwise, 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:


     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                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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       |                          H.261 stream ...                     .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The H.261 header is defined as 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |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.

   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



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   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 MC.  HMVD is encoded
   as a 2's complement number, and `10000' corresponding to the value
   -16 is forbidden (motion vector fields range from +/-15).

   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 2's 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 bit stream was decoded.  Therefore, these bits cannot
   change for the duration of the stream.  A conformant 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 fully decompress the stream, so this requires relatively
   little processing; example 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



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   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 transmission errors.  Using UDP, no mechanism is
   available at the sender to know if 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 which 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 a MB is 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), or 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
   RFC2032 SHALL not be used to request image refreshment.  Old
   implementations are encourage to use the methods describes 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
   control packets as described in



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   draft-ietf-avt-rtcp-feedback[rtcp-feedback].  The second method is by
   using the application protocol specific commands like H.245[ITU.H245]
   FastUpdateRequest.
















































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6.  Payload Format Parameters

   This section updates the H.261 media type described in RFC3555
   [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)[RFC2327] is also provided for those
   applications that use SDP.  Multiple parameters SHOULD be expressed
   as a MIME media type string, in the form of a semi-colon separated
   list of parameters.

6.1  IANA Considerations

   This section describes the MIME types and names associated with this
   payload format.  The section registers the MIME types, as per
   RFC2048[RFC2048]

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
   value 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 value are integer values 1 to 4 and it means that the
   maximum rate is 29.97/ specified value

   D: specifies support for still image graphics according to H.261
   annex D.

   Encoding considerations:

   This type is only defined for transfer via RTP [RFC3550]

   Security considerations: See Section 8



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   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 almost in every
   H.261 based video application.

   Published specification: RFC yyy

   Applications which use this media type:

   Audio and video streaming and conferencing tools.

   Additional information: none

   Person and email address to contact for further information :

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

   Intended usage: COMMON

   Author/Change controller:

   Roni Even

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 semi-colon
   separated list of parameters

6.2.1  Usage with the SDP Offer Answer Model

   When offering H.261 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



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   this SDP message is able to decode this option.  This option shall be
   considered as a receiver's capability even when send 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.
   A terminal shall announce only those picture sizes (with their MPIs)
   which it is willing to receive.  For example, MPI=2 means that
   maximum (decodeable) picture rate per second is about 15.

   If the receiver does not specify the picture size /MPI optional
   parameter then it SHOULD be ready to receive QCIF resolution with
   MPI=1.

   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


























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7.  Backward Compatibility to RFC2032

   The current draft updates RFC2032.  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 new RFC suggest generic methods to address the
   same requirement.  The authors of the drafts are not aware of
   products that supports these control packets.  Since these are
   optional features new implementations SHALL ignores them and they
   SHALL not be used by new implementations.

7.2  New SDP optional parameters

   The draft adds new optional parameters to the H261 payload type.
   Since these are optional parameters we expect old implementation to
   ignore these parameters while new implementations that will receive
   the H261 payload type with no parameters will behave as if the other
   side can accept 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 any appropriate RTP profile (for example
   [RFC3551]).  This implies that confidentiality of the media streams
   is achieved by encryption.  Because the data compression used with
   this payload format is applied end-to-end, encryption may 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 which are complex to decode and cause the receiver to
   be overloaded.  The usage of authentication of at least the RTP
   packet is RECOMMENDED

   As with any IP-based protocol, in some circumstances a receiver may
   be overloaded simply by the receipt of too many packets, either
   desired or undesired.  Network-layer authentication may be used to
   discard packets from undesired sources, but the processing cost of
   the authentication itself may be too high.




























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9.  Acknowledgements

   This is to acknowledge the authors of RFC2032 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.













































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10.  Changes from previous versions

10.1  changes from RFC 2032>

   The changes from the RFC 2032 are:

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

   Added optional parameters to the H.261 MIME type

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

10.2  Changes from bis-00

   The changes from bis-00 are:

   1.  Deprecated the H.261 specific control packets

   2.  Change the separator in the SDP parameters to be semi-colon.

10.3  Changes from bis-04

   Change to fix an error in section 6.2.1 in the SDP.  Was QCIF=3
   instead of QCIF=1



























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11.  References

11.1  Normative References

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

   [RFC2048]  Freed, N., Klensin, J. and J. Postel, "Multipurpose
              Internet Mail Extensions (MIME) Part Four: Registration
              Procedures", BCP 13, RFC 2048, November 1996.

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

   [RFC2327]  Handley, M. and V. Jacobson, "SDP: Session Description
              Protocol", RFC 2327, April 1998.

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

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

   [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.

   [rtcp-feedback]
              Ott, J. and S. Wenger, "Extended RTP Profile for
              RTCP-based Feedback(RTP/AVPF)",
              draft-ietf-avt-rtcp-feedback-08 (work in progress),
              January 2004.

11.2  Informative References

   [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.

   [H221]     International Telecommunications Union, "Frame structure
              for a 64 to 1920 kbit/s channel in audiovisual



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              teleservices", ITU Recommendation H.221, May 1999.

   [IEEE]     Pingali, S., Towsley, D. and JF. Kurose, "A comparison of
              sender-initiated and receiver-initiated reliable multicast
              protocols", IEEE GLOBECOM 94, 1994.

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

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

   [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".

   [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 !994.

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


Author's Address

   Roni Even
   Polycom
   94 Derech Em Hamoshavot
   Petach Tikva  49130
   Israel

   EMail: roni.even@polycom.co.il












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