Negotiation of Generic Image Attributes in SDP
draft-ietf-mmusic-image-attributes-03

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Abstract

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 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.

Requirements Language

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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

Table of Contents

1.  Introduction
2.  Conventions, Definitions and Acronyms
3.  Defintion of Attribute
    3.1.  Requirements
    3.2.  Attribute syntax
        3.2.1.  Overall view of syntax
        3.2.2.  Syntax description
    3.3.  Considerations
        3.3.1.  No imageattr in 1st offer
        3.3.2.  Asymmetry
        3.3.3.  sendonly and recvonly
        3.3.4.  Sample aspect ratio
        3.3.5.  SDPCapNeg support
        3.3.6.  Interaction with codec parameters
        3.3.7.  Change of display in middle of session
        3.3.8.  Use with layered codecs
        3.3.9.  Addition of parameters
4.  Examples
    4.1.  Example 1
    4.2.  Example 2
    4.3.  Example 3
    4.4.  Example 4
5.  IANA Considerations
6.  Security Considerations
7.  Acknowledgements
8.  Changes
9.  References
    9.1.  Informative References
    9.2.  Normative References
§  Authors' Addresses

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 for instance a hand-held terminal. As an example it may be beneficial to display a video image on a part of the physical screen and leave space on the screen for other features such as menus and other info.

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

• 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.
• Reduced complexity: Image rescaling can be quite computation intensive. For low end devices this can be a problem.
• 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.
• Memory requirement: The receiver device will know the size of the image and can then allocate memory accordingly.
• Optimal aspect ratio: The indication of the supported image sizes and aspect ratio allows the receiver to select the most appropriate combination based on its rescaling capabilities and the desired rendering. For example, if a sender proposes three resolutions in its SDP offer, 100x200, 200x100 and 100x100 with sar=1.0 (1:1) etc. then the receiver can select the option that fits the receiver screen best.

In cases where rescaling is not implemented (for example, rescaling is not mandatory to implement in H.264), the indication of the image attributes may still provide an optimal use of bandwidth because the attribute will anyway give the encoder a better indication about what image size is preferred and will thus help to avoid wasting bandwidth by encoding with an unnecessarily large resolution.

For implementers that are considering rescaling issues, it is worth notice note that there are several benefits to doing it on the sender side:

• 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 YUV domain and should not depend on the codecs used.
• The encoder may be able to encode in a number of formats but may not know which format to choose as it, without the image attribute, does not know the receivers performance or preference.
• 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.
• 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 the 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".

The draft is limited to unicast scenarios in general and more specific poit-to-point communication. The attribute may be used in centralized conferencing scenarios as well but due to the abundance of configuration options it may then be difficult to come up with a configuration that fits all parties.

2.  Conventions, Definitions and Acronyms

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

3.  Defintion of Attribute

This section defines the SDP image attribute "imageattr" that can be used in an SDP Offer/Answer exchange to indicate various image attribute parameters. In this document, we define the following image attribute parameters: image resolution, sample aspect ratio (sar), allowed range in picture aspect ratio and the preference of a given parameter set over another. The attribute is however extensible and guidelines for defining extensions are provided in Section 3.3.9 (Addition of parameters).

3.1.  Requirements

The image attribute MUST meet the following requirements:

REQ-1:

Support the indication of one or more set(s) of image attributes that the SDP endpoint wish to receive or send. An image attribute set MUST include a specific image size.

REQ-2:

Support setup/negotiation of image attributes, meaning that each side in the Offer/Answer SHOULD be able to negotiate the image attributes if prefers to send and receive.

REQ-3:

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

REQ-4:

Make the attribute generic with as little codec specific details/tricks as possible in order to be codec agnostic.

Besides the above mentioned requirements, the requirement below MAY be applicable.

OPT-1:

The image attribute SHOULD support the description of image-related attributes for various types of media, including video, pictures, images, etc.

3.2.  Attribute syntax

In this section the syntax of the image attribute is described. The image attribute is a media attribute. 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 in ABNF (Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” October 2005.) [RFC4234]:

    ----
    image-attr = "imageattr:" PT 1*2( 1*WSP ( "send" / "recv" )
                                      1*WSP attr-list )


    PT = 1*DIGIT / "*"
    attr-list = ( set *(1*WSP set) ) / "*"

    ; see below for a definition of set.
    ----

• Maximum one occurrence of the "send" keyword and corresponding attr-list is allowed per image attribute.
• Maximum one occurrence of the "recv" keyword and corresponding attr-list is allowed per image attribute.
• PT is the payload type number, it can be set to * to indicate that the attribute applies to all payload types in the media description.
• For sendonly or recvonly streams one of the directions MAY be omitted. See Section 3.3.3 (sendonly and recvonly), moreover the order of the send and recv directions is not important.

The syntax for the set is given by:

    ----
    set= "[" "x=" range "," "y=" range [ ",sar=" range ]
          [ ",par=" range ] [ ",q=" value ] "]"
     x is the horizontal image size range
     y is the vertical image size range
     sar (sample aspect ratio) is the sample aspect ratio associated
      with the set (optional and MAY be ignored)
     par (picture aspect ratio) is the allowed ratios between the
      displays x and y physical size (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 formats
      1) range= value
           a single value
      2) range= "[" value1 ":" [ step ":" ] value2 "]"
           values between value1 and value2 inclusive,
           if step is omitted a stepsize of 1 is implied
      3) range= "[" value 1*( "," value ) "]"
           any value from the list of values
      4) 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
      Real values are only applicable for the
       sar, par and q parameters
      Note the use of brackets [..] if more that one value
       is specified.
    ----

Some further guidelines for the use of the attribute is given below:

• 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 for instance for different video codec alternatives conditioned that the payload type number differs.
• 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.
• 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.
  If this parameter is not present a default sar value of 1.0 is assumed.
  The interpretation of sar differs between the send and the receive 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 (Sample aspect ratio) for a more detailed discussion.
  The sar parameter will likely not solve all the issues that are related to different sample aspect ratios but it can help to solve them and reduce aspect ratio distortion.
• 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

      ----
      par=[ratio_min-ratio_max]
      ----

  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.
• The offerer MUST be able to support the image attributes that it offers.
• The answerer MAY choose to keep imageattr but is not required to do so. If the attribute is kept in the SDP answer:
  • The answerer MUST for its receive direction only include one or more valid entries taken from the offer. In other words, the answerer MUST for its receive direction only pick one or more valid entries from the multidimensional solution space spanned by the offer.
  • The answerer MAY for its send direction modify the attribute in the sense that new entries other than those presented in the offer are added. It must however be noted that this may lead to an extra offer/answer exchange of the added parameters are not supported by the offerer.

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) and double-quotes 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 high level syntax the image attribute may then look like

    ----
    a=imageattr:PT send attr-list recv attr-list
    ----

or

    ----
    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:

    ----
    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:

• Provide with another list of options (attr-list). The answer from Bob may then look like:

      ----
      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 like:

      ----
      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.
• 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.

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 implications:

• Longer session setup time due to extra offer/answer exchanges
• 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. Note however that the asymmetry issue for the H.264 codec is solved in [RFC3984bis] (IETF, “RTP Payload Format for H.264 Video, http://tools.ietf.org/wg/avt/draft-ietf-avt-rtp-rfc3984bis/,” .).

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 [x=720,y=576,sar=1.1]
    ----

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 [x=720,y=576,sar=[0.91,1.0,1.09,1.45]]
    ----

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

The image attribute can be used within the SDP Capability Negotiation [SDPCapNeg] (IETF, “SDP Capability Negotiation, http://tools.ietf.org/wg/mmusic/draft-ietf-mmusic-sdp-capability-negotiation,” .) framework and its use is then specified using the "a=acap" parameter. An example is

    ----
    a=acap:1 imageattr:97 send [x=720,y=576,sar=[0.91,1.0,1.09,1.45]]
    ----

For use with SDP Media Capability Negotiation extension [SDPMedCapNeg] (IETF, “SDP media capabilities Negotiation, http://tools.ietf.org/wg/mmusic/draft-ietf-mmusic-sdp-media-capabilities,” .), where it is no longer possible to specify payload type numbers, it is possible to use the parameter substitution rule, an example of this is.

    ----
    ...
    a=mcap:1 video H264/90000
    a=acap:1 imageattr:%1% send [x=720,y=576,sar=[0.91,1.0,1.09,1.45]]
    ...
    ----

Where %1% maps to media capability number 1.

3.3.6.  Interaction with codec parameters

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

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

3.3.6.1.  H.263

The payload format for H.263 is described in [RFC4629] (Ott, H., Bormann, C., Sullivan, G., Wenger, S., and R. Even, “RTP Payload Format for ITU-T Rec,” January 2007.).

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.

3.3.6.2.  H.264

The payload format for H.264 is described in [RFC3984] (Wenger, S., Hannuksela, M., Stockhammer, T., Westerlund, M., and D. Singer, “RTP Payload Format for H.264 Video,” February 2005.) and updated in [RFC3984bis] (IETF, “RTP Payload Format for H.264 Video, http://tools.ietf.org/wg/avt/draft-ietf-avt-rtp-rfc3984bis/,” .).

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.

3.3.6.3.  MPEG-4

The payload format for MPEG-4 is described in [RFC3016] (Kikuchi, Y., Nomura, T., Fukunaga, S., Matsui, Y., and H. Kimata, “RTP Payload Format for MPEG-4 Audio/Visual Streams,” November 2000.).

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.

3.3.6.4.  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:

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

3.3.7.  Change of display in middle of session

A very likely scenario is that a user switches to another phone during a video telephony call or plugs the cellphone into an external monitor. In both cases it is very likely that a renegotiation is initiated using the SIP-REFER or SIP-UPDATE methods. It is RECOMMENDED to negotiate the image size during this renegotiation.

3.3.8.  Use with layered codecs

As the image attribute is a media line attribute, its use with layered codecs cause some concern. If the layers are transported in different RTP streams the layers are specified on different media descriptions and the relation is specified using the grouping framework [GROUPING] (IETF, “The SDP Grouping Framework, http://tools.ietf.org/html/draft-ietf-mmusic-rfc3388bis-03,” .) and the depend attribute [RFC5583] (Schierl, T. and S. Wenger, “Signaling Media Decoding Dependency in the Session Description Protocol (SDP),” July 2009.). As it is not possible to specify only one image attribute for several media descriptions the solution is either to specify the same image attribute for each media description, or to only specify the image attribute for the base layer. [Ed. note, TBD].

3.3.9.  Addition of parameters

The image attribute opens up for the addition of parameters in the future. To make backwards adaptation possible; an entity that process the attribute MUST remove parameters that are not recognized before returning the attribute in the SDP answer. Addition of future parameters that are not understood by the receiving endpoint may lead to ambiguities if mutual dependencies between parameters exist, therefore addition of parameters must be done with great care.

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 [x=800,y=640,sar=1.1,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

    ----
    a=imageattr:97 recv [x=800,y=640,sar=1.1] \
                   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 [x=800,y=640,sar=1.1] \
                   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/answer

    ----
    a=imageattr:97 send [x=800,y=640,sar=1.1] \
                   recv [x=336,y=256]
    ----

Bob replies with (actually not necessary):

    ----
    a=imageattr:97 recv [x=800,y=640,sar=1.1] \
                   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 combinations.

For the recv direction (Bob->Alice) Bob is requested to provide with a list of supported image sizes

4.3.  Example 3

In this example is defined a complete SDP offer for the video media part

    ----
    m=video 49154 RTP/AVP 99
    a=rtpmap:99 H264/90000
    a=fmtp:99 packetization-mode=0;profile-level-id=42e011; \
      sprop-parameter-sets=Z0LgC5ZUCg/I,aM4BrFSAa
    a=imageattr:99 \
      send [x=176,y=144] [x=224,y=176] [x=272,y=224] [x=320,y=240] \
      recv [x=176,y=144] [x=224,y=176] [x=272,y=224,q=0.6] [x=320,y=240]
    ----

In the send direction, sprop-parameter-sets is defined for a resolution of 320x240 which is the largest image size offered in the send direction. This means that if 320x240 is selected, no additional offer/answer is necessary. In the receive direction four alternative image sizes are offered with 272x224 being the preferred choice.

The answer may look like:

    ----
    m=video 49154 RTP/AVPF 99
    a=rtpmap:99 H264/90000
    a=fmtp:99 packetization-mode=0;profile-level-id=42e011; \
      sprop-parameter-sets=Z0LgC5ZUCg/I,aM4BrFSAa
    a=imageattr:99 send [x=320,y=240] recv [x=320,y=240]
    ----

Indicating (in this example) that the image size is 320x240 in both directions. Although the offerer preferred 272x224 for the receive direction, the answerer might not be able to offer 272x224 or not allow encoding and decoding of video of different image sizes simultaneously. The answerer sets new sprop-parameter-sets, constructed for both send and receive directions at the restricted conditions and image size of 320x240.

4.4.  Example 4

This example illustrates in more detail how compensation for different sample aspect ratios can be negotiated with the image attribute.

We setup a session between Alice and Bob, Alice is the offerer of the session. The offer (from Alice) contains the image attribute below:

    ----
    a=imageattr:97 \
      send [sar=[1.0-1.3],x=400:16:800],y=[320:16:640],par=[1.2-1.3]] \
      recv [sar=1.1,x=800,y=600]
    ----

First we consider the recv direction: The offerer (Alice) explicitly states that she wish to receive the screen resolution 800x600, however she also indicates that the screen on her display does not use square pixels, the sar value=1.1 means that Bob must (preferably) compensate for this. So.. If Bob's video camera produces square pixels, and wish to satisfy Alice's sar requirement, the image processing algorithm must rescale a 880x600 pixel image (880=800*1.1) to 800x600 pixels (could be done other ways).

... and now the send direction: Alice indicates that she can (in the image processing algorithms) rescale the image for sample aspect ratios in the range 1.0 to 1.3. She can also provide with a number of different image sizes (in pixels) ranging from 400x320 to 800x640. Bob inspects the offered sar and image sizes and responds with the modified image attribute

    ----
    a=imageattr:97 \
      recv [sar=1.15,x=464,y=384] \
      send [sar=1.1,x=800,y=600]
    ----

Alice will, in order to satisfy Bob's request, need to rescale the image from her video camera from 534x384 (534=464*1.15) to 464x384.

Neither part is required to rescale like this (sar MAY be ignored), the consequence will of course be a distorted image.

5.  IANA Considerations

Following the guidelines in [RFC4566] (Handley, M., Jacobson, V., and C. Perkins, “SDP: Session Description Protocol,” July 2006.), the IANA is requested to register one new SDP attribute:

• Contact name, email address and telephone number: Authors of RFCXXXX
• Attribute-name: imageattr
• Type of attribute: media-level
• Subject to charset: no

This attribute defines the ability to negotiate various image attributes such as image sizes. The attribute contains a number of parameters which can be modified in and offer/answer exchange.

Note to RFC Editor: please replace "RFC XXXX" above with the RFC number of this memo, and remove this note.

6.  Security Considerations

This draft does not add any additional security issues other than those already existing with currently specified offer/answer procedures.

7.  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.

8.  Changes

The main changes are:

From WG -02 to WG -03

• Partial update based on review comments from Jean-Francois Mule

From WG -01 to WG -02

• Added extra example that highlights the negotiation of sar

From WG -00 to WG -01

• Added info about future addition of parameters and backwards compatibility
• Added IANA considerations

From individual -02 to WG -00

• Cleanup of syntax, ABNF form
• Additional example

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 discussed

9.  References

9.1. Informative References

9.2. Normative References

Authors' Addresses