InfoMagic Standards 1994 January/InfoMagic Standards - January 1994.iso/inet/ietf/avt/93mar.min

Text File | 1993-05-13 | 17KB | 393 lines

CURRENT_MEETING_REPORT_

Reported by Steve Casner/Information Sciences Institute

Minutes of Audio/Video Transport Working Group (AVT)

The AVT Working Group met for three sessions.  The first two sessions
were used to discuss some open issues on the Draft specification for the
Real-Time Transport Protocol (RTP); the third session was an
``implementors agreement'' session focusing on software video encoding.

Status of the Working Group and Review of the Draft RTP Specification

The goal of the AVT Working Group is to specify a set of experimental
protocols for real-time transmission of audio and video.  The emphasis
is short-term to promote experimentation now so that standards-track
protocols can be developed based on what is learned.  The first Working
Group meeting was a year ago, so one might expect a conclusion soon.  In
fact, many issues were resolved during the last two IETF meetings and
the specification is nearing readiness for submission as an RFC.

A set of four Internet-Drafts specifying the RTP protocol were issued in
December 1992, and an Internet-Draft on packetization of H.261 coded
video was issued in March, 1993.  In addition, RTP has been implemented
to varying degrees in the following programs:


IVS              Thierry Turletti and Christian Huitema

Maven            Charley Kline

NEVOT            Henning Schulzrinne

``nv''           Ron Frederick

PictureWindow    Paul Milazzo and Bob Clements (an older draft)

PVP              Steve Casner


This session began with a brief review of RTP. It consists primarily of
protocol headers for real-time data packets, which is just eight octets
long in the typical case.  RTP supports the following functions:


   o Transfer of media data.
   o Demultiplexing of multiple flows.
   o Content identification (e.g., the media encoding used).
   o Synchronization and sequencing.
   o Options for simple control functions such as identification of
     participants.

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Full details on the protocol are available in the Internet-Drafts, which
are listed later in this document.

Discussion of Open Issues

A few open issues had developed since the last meeting and were
discussed.  The primary items were:


   o Elimination of IPv4 addresses carried within RTP.
   o Separation of RTCP control functions not related to transport.
   o Security services and mechanisms.


These issues are expanded in the following paragraphs.  No roadblocks
were identified, but resolution of some of the questions was left to
email discussion.

The CSRC and SSRC options in the December RTP specification carried
globally unique identifiers for the ``content source'' and
``synchronization source'' respectively, in data packets that have been
processed through transport/application-level gateways such as audio
mixers.  These globally unique identifiers were based on IPv4 addresses,
but this is not acceptable considering the impending transition to a
next-generation IP. Therefore, the Group considered two revision
choices:


  1. Use a (type, length, value) triple to allow any form of address to
     be carried.
  2. Carry only a 16-bit or 32-bit identifier that is locally unique to
     the gateway that inserts the option, and then define the mapping to
     a globally unique address, or other information, through an
     extended SDESC (source description) option or a higher-layer
     protocol.


Since the CSRC and SSRC options may be carried in every data packet for
some flows, the long addresses in the first choice might impose an
uncomfortably large overhead.  Furthermore, the Group recognized that
the locally unique identifier is sufficient for an RTP receiver to
distinguish the source and process the packet; it is only necessary to
map to a globally unique address or user name for purposes of
monitoring, control, and user interface.  Therefore, the second choice
was selected, and it was generally agreed that 16 bits would be
sufficient because the identifiers are unique only with respect to the
full source address of the gateway (network address plus UDP port, for
example).

Mapping of the identifiers to other information, such as the name of a
conference participant, should be accomplished through a higher-layer
control protocol.  Note that the gateway entities must be involved in
that protocol, but this would be true even if globally unique addresses

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were used since the addresses include port numbers to allow more than
one entity per host.  One way to accomplish the mapping is through the
Source Description (SDESC) option in the RTP control ``sub-protocol''
RTCP. The SDESC option includes the 16-bit identifier plus one or more
items to which it is mapped.  Examples include the user name, as before,
and various forms of globally unique addresses.

A full address must also be specified for the return of RTP reverse
control packets containing options such as the Quality of Service
Measurement (QOS) option.  In the December Draft, a return port number
was specified in the Content Description (CDESC) option, but it was
agreed that a full address is required because the return information
may go to the content or synchronization source or to third party
monitoring host.  Furthermore, it was suggested that the return address
be separated into its own option because it may be desirable to
establish a return address without defining a new content type or
redefining an old one.  One problem is that the sender and receiver of
the reverse control information may be using different forms of
addressing, for example IPv4 and SIP or PIP. This problem extends beyond
RTP, and its solution will depend on IPng transition plans.  Two
solutions the Group has considered are to use a DNS name or to allow
multiple forms of binary address to be specified.

Some members of the Working Group objected to including within the RTP
specification those RTCP options unnecessary for transport functions.
However, it seems impractical to split off a few pages of RTCP option
definitions into another RFC. It was agreed to keep the RTCP options
within the RTP specification as a separate section with appropriate
disclaimers about these functions being replaced by higher-layer control
protocols when they are available.

The RTP specification Draft includes a brief Security Considerations
section, but the protocol will be inadequate for teleconferencing
applications, at least for confidentiality of voice communication,
without access to some security mechanisms.  In the future, it may be
possible to depend on security mechanisms at the IP layer, but for
near-term use, possibly including experimentation with new security
mechanisms specifically for real-time applications, the Group believes
it is appropriate to define security mechanisms at the RTP layer.

Stuart Stubblebine made a presentation reviewing the security services
and mechanisms that might be needed for applications that use RTP. The
most needed services are confidentiality and integrity of the payload,
and authentication of the source.  Integrity is considered separately
for individual packets and for the stream of packets.  A set of three
new RTP options was proposed to implement these security services:


   o ENC, to indicate the start point for encryption and carry the
     initialization parameters;

   o MIC, which may be used with a variety of security schemes, for
     example to carry a Message Integrity Check; and


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   o KDEF, an RTCP option to define key identifiers.


Details of these security options will be found in a new Draft of the
RTP specification to be released in mid-May.

In addition to the major topics, the Group also discussed a request from
Frank Hoffmann for two additions to the protocol to allow higher
reliability levels of service.  The first was that a checksum option be
provided for use when RTP is carried over ST-II, which does not provide
a checksum.  However, there is a problem with carrying a checksum in an
option:  an error in the header may make it appear that there is no
checksum option, so the checksum would not be checked and the error
would not be detected.  Also, it is inconvenient that an RTP-level
reflector from ST-II to IP/UDP would have to check and remove the
checksum option to avoid presenting the UDP destination with two
potentially conflicting checksums.  As an alternative, it is suggested
that there be a separate specification for an encapsulation of RTP in
ST-II that would include a checksum.  That encapsulation might define a
service like that of UDP over IP.

The second request was for an option to request retransmission to
implement a ``reliable'' class of service.  It is expected that most
real-time applications will not want to incur the delay imposed by
retransmission.  A generic retransmission function probably does not
make sense in RTP, but reverse control options can be used to request
retransmission in an application-specific manner when appropriate.  For
example, negative acknowledgements are defined in the INRIA H.261
packetization protocol Draft.  For those applications that want to use
the services of RTP but do require reliable delivery, RTP can be
transported in a TCP stream.

One final topic that the Group did not have time to discuss adequately
was how RTP profiles should be defined and used.  Would it be reasonable
to include as part of a profile definition that the RTP framing field
would always be included in order to allow multiple RTP PDUs to be
assembled into, for example, one UDP packet?  The Group has also not
defined how the selection of a particular profile will be identified for
applications that can operate with more than one profile.  More work is
needed on this topic.

``Implementors Agreement'' Session

The third Working Group session was devoted to an ``implementors
agreement'' discussion to promote convergence and interoperation among
the software video compression programs being developed.

At the previous meeting, it was observed that the tight coupling between
the video frame grabbing procedure and the encoding process might mean
that it would be infeasible to define an API between these two steps.
However, Ron Frederick has observed that the coupling between software
decoding and the display process can be much more flexible.  Therefore,
it may be feasible to achieve interoperation by allowing each hardware

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and software platform to encode and transmit data in its native format,
but to incorporate multiple decode routines using a common API so that
each program can decode many or all of the other programs' native
formats.

Ron Frederick gave a presentation on the implementation of a decoder API
in version 3.0 of the ``nv'' program.  Decoding and rendering of the
image data and decoupled:  ``nv'' does all the network I/O, RTP
processing, and X window system interaction; the image decode routines
just convert each packet of compressed bits into uncompressed pixels for
a portion of the image.  When the ``S'' bit (end of synchronization
unit) is set in the RTP header, ``nv'' will display the new image on the
screen.  This scheme allows support for multiple display depths,
brightness/contrast mapping, and image scaling with simultaneous display
of multiple sizes.  Three decoders have been incorporated into ``nv'' to
process video from ``nv'', from the hardware Bolter codec, and from the
Cornell CU-SeeMe program.

Christian Huitema identified a problem with the ``nv'' API when he
described the H.261 encoding in the IVS program and the complexity that
results from a combination of image structure and Huffman encoding of
the image coefficients.  There is a lot of state information implicit in
the decoding process in the middle of processing ``group of blocks''
(GOB). Since one GOB may occupy more than one packet, it might be
infeasible to try to save and restore the state at the boundary between
packets so that control could be returned across the API from a decode
routine.  Therefore, IVS uses the ``S'' bit to indicate the GOB boundary
so that all of the packets of a GOB can be handed together to the decode
routine.

This conflict in the interpretation of the ``S'' bit will have to be
resolved to allow interoperation of ``nv'' and IVS. It was suggested
that IVS is using the ``S'' bit for a transport function, whereas ``nv''
is using it for a presentation function, and therefore the former is
correct.  Ron said ``nv'' could be modified to get the display-image
indication as a return valued from the decode routine, so this may be
the solution.

Paul Milazzo has implemented color video transmission in the BBN
PictureWindow program.  Paul proposed that the YCrCb color encoding from
the CCIR 601 digital video standard be used as the color representation
for software encoding.  This is similar to the YUV analog encoding.  The
proposed scheme would keep the luminance (Y) pixels in one array, and
chrominance (CrCb or UV) pixel pairs, subsampled by 2 in the X
dimension, in another array.  Because of the subsampling, the two arrays
would be the same size.  This scheme allows easy rendering into
monochrome or color images.

Further Working Group Activities

A set of Internet-Drafts on RTP was issued in December 1992:



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   o draft-ietf-avt-rtp-00.txt
   o draft-ietf-avt-encoding-00.txt
   o draft-ietf-avt-profile-00.txt
   o draft-ietf-avt-issues-00.ps, .txt


The first Draft is the specification of the real-time transport protocol
itself.  The second and third Drafts define a set of media encodings and
a sample profile for use of those encodings to implement audio and video
multiparticipant conferences with minimal control.  The last Draft is an
updated discussion of the issues and decisions involved in the design of
the protocol.

Revised Drafts incorporating changes discussed at this meeting will be
issued in May.  After review and possible further revision, it is
expected that the Internet-Drafts will be submitted for approval as RFCs
in June, completing the Working Group's Charter.

Attendees

Lou Berger               [e-mail]
Larry Blunk              [e-mail]
John Boatright           [e-mail]
Erik-Jan Bos             [e-mail]
Monroe Bridges           [e-mail]
Sandy Bryant             [e-mail]
Stephen Casner           [e-mail]
Yee-Hsiang Chang         [e-mail]
William Chimiak          [e-mail]
Richard Cogger           [e-mail]
David Conklin            [e-mail]
Simon Coppins            [e-mail]
Mark Davis-Craig         [e-mail]
Shane Dawalt             [e-mail]
Steve DeJarnett          [e-mail]
Tony DeSimone            [e-mail]
Ed Ellesson              [e-mail]
Chip Elliott             [e-mail]
Hans Eriksson            [e-mail]
Francois Fluckiger       [e-mail]
Paul Franchois           [e-mail]
Ron Frederick            [e-mail]
Jerry Friesen            [e-mail]
Marcello Frutig          [e-mail]
Gwen Funchess            [e-mail]
Joseph Godsil            [e-mail]
Fengmin Gong             [e-mail]
Kenneth Goodwin          [e-mail]
Mark Green               [e-mail]
Robert Gutierrez         [e-mail]
Don Hoffman              [e-mail]
Frank Hoffmann           [e-mail]
Christian Huitema        [e-mail]

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Phil Irey                [e-mail]
Ronald Jacoby            [e-mail]
Peter Kirstein           [e-mail]
Charley Kline            [e-mail]
Lakshman Krishnamurthy   [e-mail]
Giri Kuthethoor          [e-mail]
Paul Lambert             [e-mail]
Ruth Lang                [e-mail]
Ronald Lanning           [e-mail]
Yu-Lin Lu                [e-mail]
Marjo Mercado            [e-mail]
Donald Merritt           [e-mail]
Paul Milazzo             [e-mail]
Joseph Pang              [e-mail]
Geir Pedersen            [e-mail]
Jim Rees                 [e-mail]
Michael Safly            [e-mail]
Carl Schoeneberger       [e-mail]
Eve Schooler             [e-mail]
Stuart Stubblebine       [e-mail]
Sally Tarquinio          [e-mail]
Craig Todd               [e-mail]
Claudio Topolcic         [e-mail]
Hung Vu                  [e-mail]
Huyen Vu                 [e-mail]
Abel Weinrib             [e-mail]
John Wroclawski          [e-mail]
Yow-Wei Yao              [e-mail]



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