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Versions: 00 01 02 draft-ietf-mmusic-image-attributes

Network Working Group                                       I. Johansson
Internet-Draft                                               Ericsson AB
Intended status: Standards Track                                 K. Jung
Expires: March 23, 2009                    Samsung Electronics Co., Ltd.
                                                            Sep 19, 2008

             Negotiation of Generic Image Attributes in SDP

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This document proposes a new generic session setup attribute to make
   it possible to negotiate different image attributes such as image
   size.  A possible use case is to make it possible for a e.g a low-end
   hand-held terminal to display video without the need to rescale the
   image, something that may consume large amounts of memory and
   processing power.  The draft also helps to maintain an optimal
   bitrate for video as only the image size that is desired by the
   receiver is transmitted.

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Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions, Definitions and Acronyms  . . . . . . . . . . . .  4
   3.  Defintion of Attribute . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Attribute syntax . . . . . . . . . . . . . . . . . . . . .  5
       3.2.1.  Overall view of syntax . . . . . . . . . . . . . . . .  5
       3.2.2.  Syntax description . . . . . . . . . . . . . . . . . .  7
     3.3.  Considerations . . . . . . . . . . . . . . . . . . . . . .  9
       3.3.1.  No imageattr in 1st offer  . . . . . . . . . . . . . .  9
       3.3.2.  Asymmetry  . . . . . . . . . . . . . . . . . . . . . .  9
       3.3.3.  sendonly and recvonly  . . . . . . . . . . . . . . . . 10
       3.3.4.  Sample aspect ratio  . . . . . . . . . . . . . . . . . 10
       3.3.5.  SDPCapNeg support  . . . . . . . . . . . . . . . . . . 10
       3.3.6.  Interaction with codec parameters  . . . . . . . . . . 10
   4.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  Changes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Informative References . . . . . . . . . . . . . . . . . . 15
     10.2. Normative References . . . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
   Intellectual Property and Copyright Statements . . . . . . . . . . 17

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1.  Introduction

   This document proposes a new attribute to make it possible to
   negotiate different image attributes such as image size.  The term
   image size is defined here as it may differ from the physical screen
   size of e.g a hand-held terminal.  For instance it may be beneficial
   to display a video image on a part of the physical screen and leave
   space on the screen for e.g menus and other info.

   There are a number of benefits with a possibility to negotiate the
   image size:

   o  Less image distortion: Rescaling of images introduces additional
      distortion, something that can be avoided (at least on the
      receiver side) if the image size can be negotiated.

   o  Reduced complexity: Image rescaling can be quite computation
      intensive.  For low end devices this can be a problem.

   o  Optimal quality for the given bitrate: The sender does not need to
      encode an entire CIF (352x288) image if only an image size of
      288x256 is displayed on the receiver screen.  This gives
      alternatively a saving in bitrate.

   o  Memory requirement: The receiver device will know the size of the
      image and can then allocate memory accordingly.

   o  Optimal aspect ratio: If rescaling of the image is possible on the
      received side one can imagine that the offer contains three
      resolutions 100x200, 200x100 and 100x100 with sar=1.0 (1:1).  If
      the receiver screen has the resolution 200x200 with sar=1 then the
      obvious is to select 100x100 and scale the image by a factor 2.
      If on the other hand the screen has the resolution 100x200 with
      sar=2 (2:1) then the obvious is again to select 100x100 and scale
      the image by a factor 2 in the x-axis.

   The cautious reader may however object that the rescaling issue has
   been moved to the sender and also that codecs such as H.264 are not
   mandated to support the rescaling of the video image size.  This
   potentially reduces the number of valid arguments to only 1 (optimal
   use of bandwidth).

   However, what must then be considered is that:

   o  Rescaling on the sender/encoder side is likely to be easier to do
      as the camera related software/hardware already contains the
      necessary functionality for zooming/cropping/trimming/sharpening
      the video signal.  Moreover, rescaling is generally done in RGB or

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      YUV domain and should not depend on the codecs used.

   o  The encoder may be able to encode in a number of formats but may
      not know which format to choose.

   o  The quality drop due to digital domain rescaling using
      interpolation is likely to be lower if it is done before the video
      encoding rather than after the decoding esp. when low bitrate
      video coding is used.

   o  If low-complexity rescaling operations such as cropping only must
      be performed after all, the benefit with having this functionality
      on the sender side is that it is then possible to present a
      miniature "what you send" image on the display to help the user to
      target his camera correctly.

   Several of the existing standards ([H.263], [H.264] and [MPEG-4])
   have support for different resolutions at different framerates.  The
   purpose of this document is to provide for a generic mechanism and is
   targeted mainly at the negotiation of the image size but to make it
   more general the attribute is named "imageattr".  A problem statement
   and discussion that gives a motivation to this document can be found
   in [S4-080144].

2.  Conventions, Definitions and Acronyms

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

3.  Defintion of Attribute

   A new image attribute is defined with the name "imageattr".  The
   image attribute contains a set of different image-resolution options
   that the offerer can provide.  The receiver can then select the
   desired image attribute (e.g image size) and may then have the
   ability to avoid costly transformations (e.g rescaling) of the
   images.  In this approach only the image resolution and optionally
   sample aspect ratio, allowed range in picture aspect ratio and
   preference is covered but the framework makes it possible to extend
   with other image related attributes that make sense.

3.1.  Requirements

   The new image attribute should meet the following requirements:

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   REQ-1:  Support the offer of a specific image size on the receiver
      display or in other words, reduce or avoid the need for rescaling
      images in the receiver to fit a given portion of the screen.

   REQ-2:  Support asymmetric setups i.e the very likely scenario where
      Alice prefers an image size of 320x240 for her display while Bob
      prefers an image size of 176x144.

   REQ-3:  Interoperate with codec specific parameters such as sprop-
      parameter-sets in H.264 or config in MPEG4.

   REQ-3:  Make the attribute generic with as little codec specific
      details/tricks as possible.  Ideally the attribute should not care
      about the codec specific features.

   OPT-1:  Make it possible to use attribute for other purposes than
      video.  One possible use case may be distributed white-board
      presentations which are based on transmission of compressed bitmap
      images where rescaling often produce very poor results.

3.2.  Attribute syntax

   In this section the syntax of the image attribute is described.  The
   section is split up in two parts, the first gives an overall view of
   the syntax while the second describes how the syntax is used.

3.2.1.  Overall view of syntax

   The syntax for the image attribute is:
       a=imageattr:PT 1*WSP send 1*WSP attr_list \
                      1*WSP recv 1*WSP attr_list|*
       attr_list = set *(1*WSP set)
        see below for a definition of set.

   o  PT is the payload type number, can be set to * .

   o  The backslash '\' is used in this document for formatting reasons

   o  For sendonly or recvonly streams one of the directions MAY be
      omitted.  See Section 3.3.3, moreover the order of the send and
      recv directions is not important.

   The syntax for the set is given by:

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     x is the horizontal image size range
     y is the vertical image size range
     sar is the sample aspect ratio associated with the set
      (optional and MAY be ignored)
     par is the allowed ratios between the displays x and y physical size
       a.k.a picture aspect ratio (optional)
     q (optional with range [0.0..1.0], default value 0.5)
       is the preference for the given set, a higher value means higher
       preference from the sender point of view

     range is expressed in a few different forms
      1) range=value
      2) range=[value1:value2]
           (any value between value1 and value2 inclusive)
      3) range=[value1:step:value2]
           (every 'step' value between value1 and value2 inclusive)
      4) range=[value1,value2...valueN]
           (any value from the list of values)
      5) range=[value1-value2]
           (any real value between value1 and value2 inclusive)

      value is a positive integer or real value
      step is a positive integer or real value
       If step is left out in the syntax a stepsize of 1 is implied.
      Note the use of brackets [..] if more that one value specified.

   Some guidelines for the use of the syntax is given below:

   o  The image attribute is bound to a specific media by means of the
      payload type number.  A wild card (*) can be specified for the
      payload type number to indicate that it applies to all payload
      types in the media description.  Several image attributes can be
      defined e.g for different video codec alternatives conditioned
      that the payload type number differs.

   o  The preference for each set is 0.5 by default, setting the
      optional q parameter to another value makes it possible to set
      different preferences for the sets.  A higher value gives a higher
      preference for the given set.

   o  The sar parameter specifies the sample aspect ratio associated to
      the given range of x and y values.  The sar parameter is defined
      as dx/dy where dx and dy is the size of the pixels.  Square pixels
      gives a sar=1.0.  The parameter sar MAY be expressed as a range.
      The interpretation of sar differs between the send and the receive

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      directions.  In the send direction it defines a specific sample
      aspect ration associated to a given x and y image size (range).
      In the recv direction sar expresses that the receiver of the given
      media prefers to receive a given x and y resolution with a given
      sample aspect ratio.  See Section 3.3.4 for a more detailed

   o  The par (width/height = x/y ratio) parameter indicates a range of
      allowed ratios between x and y physical size (picture aspect
      ratio).  This is used to limit the number of x and y image size
      combinations, par is given as
      Where ratio_min and ratio_max are the min and max allowed picture
      aspect ratios.
      If sar and the display sample aspect ration is the same (or close)
      the relation between the x and y pixel resolution and the physical
      size of the image is straightforward.  If however sar differs from
      the sample aspect ratio of the receiver display this must be taken
      into consideration when the x and y pixel resolution alternatives
      are sorted out.

   o  The offerer MUST be able to support the image attributes that it

   o  The answerer MAY choose to keep imageattr but is not required to
      do so.  The answerer MUST in this case only include one or more
      valid entries taken from the offer in the SDP answer, the
      exception to this is the case above where the 1st offer defines a
      desired image size for the receive direction.

   o  The answerer MUST only pick one or more valid entries from the
      multidimensional solution space spanned by the offer.

3.2.2.  Syntax description

   In the description of the syntax we here assume that Alice wish to
   setup a session with Bob and that Alice takes the first initiative.
   The syntactical white-space delimiters (1*WSP) are removed to make
   reading easier.

   In the offer Alice provides with information for both the send and
   receive (recv) directions using syntax version 1.  For the send
   direction Alice provides with a list that the answerer can select
   from.  For the receive direction Alice may either specify a desired
   image size range right away or a * to instruct Bob to fill with a
   list of image size that Bob can support to send.  Using the overall

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   high level syntax the image attribute may then look like
       a=imageattr:PT send attr_list recv attr_list
       a=imageattr:PT send attr_list recv *
   In the first alternative the recv direction may be a full list of
   desired image size formats.  It may however (and most likely) just be
   a list with one alternative for the preferred x and y resolution.

   If Bob supports an x and y resolution in the given x and y range the
   answer from Bob will look like (syntax version 2):
       a=imageattr:PT send attr_list recv attr_list
   And the offer answer negotiation is done.  Worth notice here is that
   the attr_list will likely be pruned in the answer.  While it may
   contain many different alternatives in the offer it may in the end
   contain just one or two alternatives in the end.

   If Bob does not support any x and y resolution in the given x and y
   range in attr_list or a (*) was given for the recv direction then he
   MUST either:

   o  Provide with another list of options (attr_list).  The answer from
      Bob may then look like (syntax version 3):
       a=imageattr:PT recv attr_list send attr_list
      In this case the offer/answer negotiation is not quite done.  To
      complete the offer/answer Alice sends another offer that looks
       a=imageattr:PT send attr_list recv attr_list
      Bob MAY send back an answer to complete the 2nd offer/answer but
      this is not necessary.

   o  Remove the corresponding part completely in which case the answer
      from bob would look like:
       a=imageattr:PT recv attr_list
      Again it is worth notice that the attr_list for each direction is
      likely pruned depending on preferred and supported options.

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   If the 1st offer (from Alice) already defines a desired image size
   for the recv direction the answerer can do one of the following:

   1.  Accept the image size and return it in the answer.

   2.  Replace with a list of options in the answer.

   3.  Remove the corresponding part completely.  This may happen if it
       is deemed that it is unlikely that the list of options is
       supported.  The answer will then lack a description for the send
       direction and will look like:
       a=imageattr:PT recv attr_list

3.3.  Considerations

3.3.1.  No imageattr in 1st offer

   A high end device (Alice) may not see any need for the image
   attribute as it most likely has the processing capacity to rescale
   incoming video and may therefore not include the attribute in the
   offer as it otherwise does not see any use for it.  The answerer
   (Bob) MAY include imageattr in the answer.  This has two

   o  Longer session setup time due to extra offer/answer exchanges

   o  There is a risk that Alice does not recognize or support imageattr
      and will thus anyway ignore the attribute.

3.3.2.  Asymmetry

   While the image attribute supports asymmetry there are some
   limitations to this.  One important limitation is that the codec
   being used can only support up to a given maximum resolution for a
   given profile level.

   As an example H.264 with profile level 1.2 does not support higher
   resolution than 352x288 (CIF).  The offer/answer rules essentially
   gives that the same profile level must be used in both directions.
   This means that for an asymmetric scenario where Alice wants an image
   size of 580x360 and Bob wants 150x120 profile level 2.2 is needed in
   both directions even though profile level 1 would have been enough in
   one direction.

   Currently, the only solution to this problem is to specify two
   unidirectional media descriptions.

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3.3.3.  sendonly and recvonly

   If the directional attributes a=sendonly or a=recvonly are given for
   a media, there is of course no need to specify the image attribute
   for both directions.  Therefore one of directions in the attribute
   MAY be omitted.  However it may be good to do the image attribute
   negotiation in both directions in case the session is updated for
   media in both directions at a later stage.

3.3.4.  Sample aspect ratio

   The sar parameter in relation to the x and y pixel resolution
   deserves some extra discussion.  Consider the offer from Alice to Bob
   (we set the recv direction aside for the moment):
       a=imageattr:97 send [sar=1.1,x=720,y=576]
   If the receiver display has square pixels the 720x576 image would
   need to be rescaled to for example 792x576 or 720x524 to ensure a
   correct image aspect ratio.  This in practice means that rescaling
   would need to be performed on the receiver side, something that is
   contrary to the spirit of this draft.
   To avoid this problem Alice MAY specify a range of values for the sar
   parameter like:
       a=imageattr:97 send [sar=[0.91,1.0,1.09,1.45],x=720,y=576]
   Meaning that Alice can encode with any of the mentioned sample aspect
   ratios, leaving to Bob to decide which one he prefers.

   The response MUST not include the sar parameter if there is no
   acceptable value given.

3.3.5.  SDPCapNeg support

   T.B.D when the rest is settled.

3.3.6.  Interaction with codec parameters

   As most codecs specifies some kind of indication of eg. the image
   size already at session setup some measures must be taken to avoid
   that the image attribute conflicts with this already existing

   The following subsections describes the most well known codecs and
   how they define image-size related information.

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   The payload format for H.263 is described in [RFC4629].

   H.263 defines (on the fmtp line) a list of image sizes and their
   maximum frame rates (profiles) that the offerer can receive.  The
   answerer is not allowed to modify this list and must reject a payload
   type that contains an unsupported profile.  The CUSTOM profile may be
   used for image size negotiation but support for asymmetry requires
   the specification of two unidirectional media descriptions using the
   sendonly/recvonly attributes.  H.264

   The payload format for H.264 is described in [RFC3984].  This format
   is subject to change and the update is described in [ref. to
   RFC3984bis and other H.264 ext drafts]

   H.264 defines image size related information in the fmtp line by
   means of sprop-parameter-sets.  According to the specification
   several sprop-parameter-sets may be defined for one payload type.
   The sprop-parameter-sets describe the image size (+ more) that the
   offerer sends in the stream and need not be complete.  This means
   that this does not represent any negotiation.  Moreover an answer is
   not allowed to change the sprop-parameter-sets.

   This configuration may be changed later inband if for instance image
   sizes need to be changed or added.  MPEG-4

   The payload format for MPEG-4 is described in [RFC3016].

   MPEG-4 defines a config parameter on the fmtp line which is a
   hexadecimal representation of the MPEG-4 visual configuration
   information.  This configuration does not represent any negotiation
   and the answer is not allowed to change the parameter.

   Currently it is not possible to change the configuration using inband
   signaling.  Possible solutions

   The subsections above clearly indicate that this kind of information
   must be aligned well with the image attribute to avoid conflicts.
   There are a number of possible solutions:

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   o  Ignore payload format parameters: This may not work well e.g in
      the presence of bad channel conditions esp. in the beginning of a
      session.  Moreover this is not a good option for MPEG-4.

   o  2nd session-wide offer/answer round: In the 2nd offer/answer the
      codec payload format specific parameters are defined based on the
      outcome of the imageattr negotiation.  The drawback with this is
      that setup of the entire session (including audio) may be delayed
      considerably, especially as the imageattr negotiation can already
      itself cost up to two offer/answer rounds.  Also the conflict
      between the imageattr negotiation and the payload format specific
      parameters is still present after the first offer/anser round and
      a fuzzy/buggy implementation may start media before the second
      offer/answer is completed with unwanted results.

   o  2nd session-wide offer/answer round only for video: This is
      similar to the alternative above with the exception that setup
      time for audio is not increased, moreover the port number for
      video is set to 0 during the 1st offer answer round to avoid that
      media flows.
      This has the effect that video will blend in some time after the
      audio is started (up to 2 seconds delay).  This alternative is
      likely the most clean-cut and failsafe alternative.  The drawback
      is, as the port number in the first offer is always zero, the
      media startup will always be delayed even though it would in fact
      have been possible to start media already after the first offer/
      answer round.

4.  Examples

   A few examples to highlight the syntax, here is assumed where needed
   that Alice initiates a session with Bob

4.1.  Example 1
       a=imageattr:97 send [sar=1.1,x=800,y=640,q=0.6] [x=480,y=320] \
                      recv [x=330,y=250]

   Two image resolution alternatives are offered with 800x640 with
   sar=1.1 having the highest preference

   The example also indicates that Alice wish to display video with a
   resolution of 330x250 on her display

   In case Bob accepts the "recv [x=330,y=250]" the answer may look like

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       a=imageattr:97 recv [sar=1.1,x=800,y=640] \
                      send [x=330,y=250]

   Indicating that the receiver (Bob) wish the encoder (on Alice's side)
   to compensate for a sample aspect ratio of 1.1 (11:10) and desires an
   image size on its screen of 800x640.

   There is however a possibility that "recv [x=330,y=250]" is not
   supported.  If the case, Bob may completely remove this part or
   replace it with a list of supported image sizes.
       a=imageattr:97 recv [sar=1.1,x=800,y=640] \
                      send [x=[320:16:640],y=[240:16:480],par=[1.2-1.3]]

   Alice can then select a valid image size which is closest to the one
   that was originally desired (336x256) and performs a second offer/
       a=imageattr:97 send [sar=1.1,x=800,y=640] \
                      recv [x=336,y=256]

   Bob replies with (actually not necessary):
       a=imageattr:97 recv [sar=1.1,x=800,y=640] \
                      send [x=336,y=256]

4.2.  Example 2
       a=imageattr:97 \
            send [x=[480:16:800],y=[320:16:640],par=[1.2-1.3],q=0.6] \
                 [x=[176:8:208],y=[144:8:176],par=[1.2-1.3]] \
            recv *

   Two image resolution sets are offered with the first having a higher
   preference (q=0.6).  The x-axis resolution can take the values 480 to
   800 in 16 pixels steps and 176 to 208 in 8 pixels steps.  The par
   parameter limits the set of possible x and y screen resolution
   combinations such that 800x640 (ratio=1.25) is a valid combination
   while 720x608 (ratio=1.18) or 800x608 (ratio=1.31) are invalid

   For the recv direction (Bob->Alice) Bob is requested to provide with

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   a list of supported image sizes

5.  Issues

   Here is listed a few possible issues and observations connected to
   the use of the image attribute :

   o  Sample aspect ratios: Should it be possible to specify several
      different values for sar?

   o  interlace parameter: It should be possible to include an interlace
      parameter with the two values true and false for each set.

   o  Change of display during session: There are roughly speaking two

      *  The user plugs in an external monitor

      *  The user moves the call to another unit (SIP-REFER)

      Likely these scenarios involve some kind of renegotiation.

   o  The syntax description may need to be improved.

   o  Image sharpening: In normal cases image sharpening is to be
      performed after rescaling on the sender side, this also yields the
      best possible image quality.  There may however occur cases where
      it is preferable to do the sharpening on the receiver side
      instead.  Big question is if this document should worry about this
      as it has traditionally been up to the sender and/or receiver side
      to determine if sharpening should be applied.

   o  Multiparty sessions: Currently only point-to-point scenarios is
      considered, it is however possible that this formulation is used
      for multiparty sessions with a mixer/transcoding device in the
      middle.  It should be possible to use imageattr for the latter
      scenario but this must be studied more.

6.  IANA Considerations


7.  Security Considerations

   This draft does not add any additional security issues other than

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   those already existing with currently specified offer/answer

8.  Acknowledgements

   The authors would like to thank the people who has contributed with
   objections and suggestions to this draft and provided with valuable
   guidance in the amazing video-coding world.  Special thanks go to
   Clinton Priddle, Roni Even, Randell Jesup, and Dan Wing.

9.  Changes

   The main changes are:

   From -01 to -02

      *  Cleanup of the sar and par parameters to make them match the
         established conventions

      *  Requirement specification added

      *  New bidirectional syntax

      *  Interoperability considerations with well known video codecs

10.  References

10.1.  Informative References

   [H.264]    ITU-T, "ITU-T Recommendation H.264,

   [RFC3016]  Kikuchi, Y., Nomura, T., Fukunaga, S., Matsui, Y., and H.
              Kimata, "RTP Payload Format for MPEG-4 Audio/Visual
              Streams", RFC 3016, November 2000.

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

   [RFC3984]  Wenger, S., Hannuksela, M., Stockhammer, T., Westerlund,
              M., and D. Singer, "RTP Payload Format for H.264 Video",
              RFC 3984, February 2005.

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

   [RFC4587]  Even, R., "RTP Payload Format for H.261 Video Streams",
              RFC 4587, August 2006.

   [RFC4629]  Ott, H., Bormann, C., Sullivan, G., Wenger, S., and R.
              Even, "RTP Payload Format for ITU-T Rec", RFC 4629,
              January 2007.

              3GPP, "Signaling of Image Size: Combining Flexibility and
              Low Cost, http://www.3gpp.org/ftp/tsg_sa/WG4_CODEC/

10.2.  Normative References

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

Authors' Addresses

   Ingemar Johansson
   Ericsson AB
   Laboratoriegrand 11
   SE-971 28 Lulea

   Phone: +46 73 0783289
   Email: ingemar.s.johansson@ericsson.com

   Kyunghun Jung
   Samsung Electronics Co., Ltd.
   Dong Suwon P.O. Box 105
   416, Maetan-3Dong, Yeongtong-gu
   Suwon-city, Gyeonggi-do
   Korea 442-600

   Phone: +82 10 9909 4743
   Email: kyunghun.jung@samsung.com

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