SDP: Session Description Protocol

Internet Engineering Task Force                                   MMUSIC WG
INTERNET-DRAFT                                    Mark Handley/Van Jacobson
draft-ietf-mmusic-sdp-03.txt                                       ISI/LBNL
                                                            26th March 1997
                                                    Expires: 26th Sept 1997

                   SDP: Session Description Protocol

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     This document defines the Session Description  Protocol,  SDP.
     SDP  is  intended  for  describing multimedia sessions for the
     purposes of  session  announcement,  session  invitation,  and
     other forms of session initiation.

This document is a product of the Multiparty Multimedia Session  Control
(MMUSIC) working group of the Internet Engineering Task Force.  Comments
are solicited and should be addressed to  the  working  group's  mailing
list at and/or the authors.

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1.  Introduction

On the Internet multicast backbone (Mbone), a session directory tool  is
used  to advertise multimedia conferences and communicate the conference
addresses and conference tool-specific information necessary for  parti-
cipation.  This document defines a session description protocol for this
purpose, and for general real-time multimedia session  description  pur-
poses.  This draft does not describe multicast address allocation or the
distribution of SDP messages in detail.  These are described  in  accom-
panying  drafts.   SDP  is not intended for negotitation of media encod-

2.  Background

The Mbone is the part of the internet that supports  IP  multicast,  and
thus  permits  efficient  many-to-many communication.  It is used exten-
sively for multimedia conferencing.  Such conferences usually  have  the
property  that tight coordination of conference membership is not neces-
sary; to receive a conference, a user at an Mbone site only has to  know
the  conference's  multicast  group  address  and  the UDP ports for the
conference data streams.

Session directories assist the advertisement of conference sessions  and
communicate  the  relevant  conference  setup information to prospective
participants.  SDP is designed to convey such information to recipients.
SDP  is  purely a format for session description - it does not incorpor-
tate a transport protocol, and is intended to  use  different  transport
protocols  as  appropriate  including  the Session Announcement Protocol
[4], electronic mail using the MIME extensions, and the Hypertext  Tran-
sport Protocol.

SDP is intended to be general purpose so that it can be used for a wider
range  of network environments and applications than just multicast ses-
sion directories.  However, it is not intended to support negotiation of
session content or media encodings - this is viewed as outside the scope
of session description.

3.  Glossary of Terms

The following terms are used in this document, and have specific meaning
within the context of this document.

    A multimedia conference is a set of two or more communicating  users
    along with the software they are using to communicate.


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    A multimedia session is a set of multimedia  senders  and  receivers
    and  the  data  streams  flowing  from senders to receivers.  A mul-
    timedia conference is an example of a multimedia session.

Session Advertisement
    See session announcement.

Session Announcement
    A session announcement is a mechanism by which a session description
    is  conveyed  to  users  in  a pro-active fashion, i.e., the session
    description was not explicitly requested by the user.

Session Description
    A well defined format for conveying sufficient information  to  dis-
    cover and participate in a multimedia session.

4.  SDP Usage

4.1.  Multicast Announcements

SDP is a session description protocol for multimedia sessions.  A common
mode  of  usage  is  for  a  client  to announce a conference session by
periodically multicasting an announcement packet to a well known  multi-
cast address and port using the Session Announcement Protocol (SAP).

SAP packets are UDP packets with the following format:

         0                   31
         | SAP header         |
         | text payload       |

The  header  is  the  Session  Announcement  Protocol  header.   SAP  is
described in more detail in a companion draft [4]

The text payload is an SDP session description,  as  described  in  this
draft.   The  text  payload should be no greater than 1 Kbyte in length.
If announced by SAP, only one session announcement  is  permitted  in  a
single packet.

4.2.  Email and WWW Announcements

Alternative means of conveying session descriptions  include  electronic
mail  and  the World Wide Web.  For both email and WWW distribution, the

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use of the MIME content type ``application/sdp'' should be  used.   This
enables the automatic launching of applications for participation in the
session from the WWW client or mail reader in a standard manner.

Note that announcements of multicast sessions made only via email or the
World  Wide  Web  (WWW)  do not have the property that the receiver of a
session announcement can necessarily receive  the  session  because  the
multicast  sessions  may  be  restricted in scope, and access to the WWW
server or reception of  email  is  possible  outside  this  scope.   SAP
announcements do not suffer from this mismatch.

5.  Requirements and Recommendations

The purpose of SDP is to convey information about media streams in  mul-
timedia  sessions  to  allow  the recipients of a session description to
participate in the session.  SDP is primarily intended  for  use  in  an
internetwork,  although  it is sufficiently general that it can describe
conferences in other network environments.

A multimedia session, for these purposes, is defined as a set  of  media
streams  that  exist  for  some  duration of time.  Media streams can be
many-to-many.  The times during which the session is active need not  be

Thus far, multicast based sessions on the internet  have  differed  from
many  other  forms  of conferencing in that anyone receiving the traffic
can join the session (unless the session traffic is encrypted).  In such
an  environment, SDP serves two primary purposes.  It is a means to com-
municate the existence of a session, and is a means to convey sufficient
information  to  enable  joining and participating in the session.  In a
unicast environment, only the latter purpose is likely to be relevant.

Thus SDP includes:

o    Session name and purpose

o    Time(s) the session is active

o    The media comprising the session

o    Information to receive those media (addresses, ports,  formats  and
    so on)

As resources necessary to participate in a session may be limited,  some
additional information may also be desirable:

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o    Information about the bandwidth to be used by the conference

o    Contact information for the person responsible for the session

In general, SDP must convey sufficient information to be able to join  a
session (with the possible exception of encryption keys) and to announce
the resources to be used to non-participants that may need to know.

5.1.  Media Information

SDP includes:

o    The type of media (video, audio, etc)

o    The transport protocol (RTP/UDP/IP, H.320, etc)

o    The format of the media (H.261 video, MPEG video, etc)

For an IP multicast session, the following are also conveyed:

o    Multicast address for media

o    Transport Port for media

This address and port are the destination address and  destination  port
of the multicast stream, whether being sent, received, or both.

For an IP unicast session, the following are conveyed:

o    Remote address for media

o    Transport port for contact address

The semantics of this address and port depend on the media and transport
protocol  defined.   By  default,  this is the remote address and remote
port to send data to, and the remote address and local port for  receiv-
ing  data.   However,  some media may define to use these to establish a
control channel for the actual media flow.

5.2.  Timing Information

Sessions may either be bounded or unbounded in  time.   Whether  or  not
they are bounded, they may be only active at specific times.

SDP can convey:

o     An arbitrary list of start and stop times bounding the session

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o     For each bound, repeat times such as "every Wednesday at 10am  for
     one hour"

This timing information is globally consistent,  irrespective  of  local
time zone or daylight saving time.

5.3.  Private Sessions

It is possible to create both  public  sessions  and  private  sessions.
Private  sessions  will  typically be conveyed by encrypting the session
description to distribute it.  The details of  how  encryption  is  per-
formed  are  dependent on the mechanism used to convey SDP - see [4] for
how this is done for session announcements.

If a session announcement is private it is possible to use that  private
announcement  to  convey encryption keys necessary to decode each of the
media in a  conference,  including  enough  information  to  know  which
encryption scheme is used for each media.

5.4.  Obtaining Further Information about a Session

A session description should convey enough information to decide whether
or not to participate in a session.  SDP may include additional pointers
in the form of Universal Resources Identifiers (URIs) for more  informa-
tion about the session.

5.5.  Categorisation

When many session descriptions are being distributed by SAP or any other
advertisement  mechanism,  it  may  be desirable to filter announcements
that are of interest from those that are not.  SDP supports a  categori-
sation mechanism for sessions that is capable of being automated.

5.6.  Internationalization

The SDP specification recommends the use of 8-bit ISO  8859-1  character
sets  to  allow  the  extended ASCII characters used by many western and
northern European languages to be represented.  However, there are  many
languages  that  cannot  be  represented in an ISO 8859-1 character set.
SDP allows extensions to allow other character  sets  to  be  used  when

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6.  SDP Specification

SDP session descriptions are entirely textual.   The  textual  form,  as
opposed to a binary encoding such as ASN/1 or XDR, was chosen to enhance
portability, to enable a variety of transports to be used (e.g,  session
description  in  a MIME email message) and to allow flexible, text-based
toolkits (e.g., Tcl/Tk ) to be used to generate and to  process  session
descriptions.   However,  since the total bandwidth allocated to all SAP
announcements is strictly limited, the encoding is deliberately compact.
Also,  since  announcements may be transported via very unreliable means
(e.g., email) or damaged by an intermediate caching server, the encoding
was  designed with strict order and formatting rules so that most errors
would result in malformed announcements which could be  detected  easily
and discarded.  This also allows rapid discarding of encrypted announce-
ments for which a receiver does not have the correct key.

An SDP session description consists of a number of lines of text of  the
<type> is always exactly one character and is case-significant.  <value>
is  a structured text string whose format depends on <type>.  Whitespace
is not permitted either side of the `='  sign.  In  general  <value>  is
either  a  number  of  fields delimited by a single space character or a
free format string.

A session description consists of a session-level descriptions  (details
that  apply  to  the whole session and all media streams) and optionally
several media-level descriptions (details that apply onto  to  a  single
media stream).

An announcement consists of a session-level section followed by zero  or
more  media-level  sections.   The session-level part starts with a `v='
line and continues to the first media-level section.  The media descrip-
tion  starts  with an `m=' line and continues to the next media descrip-
tion or end of the whole session description.  In general, session-level
values  are the default for all media unless overridden by an equivalent
media-level value.

When SDP is conveyed by SAP, only one session description is allowed per
packet.   When SDP is conveyed by other means, many SDP session descrip-
tions may be concatenated together (the `v=' line indicating  the  start
of  a  session  description  terminates the previous description).  Some
lines in each description are required and some  are  optional  but  all
must  appear  in  exactly  the order given here (the fixed order greatly
enhances error detection and allows  for  a  simple  parser).   Optional
items are marked with a `*'.

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        Session description
                v=  (protocol version)
                o=  (owner/creator and session identifier).
                s=  (session name)
                i=* (session information)
                u=* (URI of description)
                e=* (email address)
                p=* (phone number)
                c=* (connection information - not required if included in all media)
                b=* (bandwidth information)
                One or more time descriptions
                z=* (time zone adjustments)
                k=* (encryption key)
                a=* (zero or more session attribute lines)
                Zero or more media descriptions

        Time description
                t=  (time the session is active)
                r=* (zero or more repeat times)

        Media description
                m=  (media name and transport address)
                i=* (media title)
                c=* (connection information - optional if included at session-level)
                b=* (bandwidth information)
                k=* (encryption key)
                a=* (zero or more media attribute lines)

The set of `type' letters is deliberately small and not intended  to  be
extensible  --  SDP parsers must completely ignore any announcement that
contains a `type' letter that it does not understand.   The  `attribute'
mechanism  (described  below) is the primary means for extending SDP and
tailoring it to particular applications or media.  Some attributes  (the
ones  listed  in this document) have a defined meaning but others may be
added on an application-, media- or session-specific basis.   A  session
directory must ignore any attribute it doesn't understand.

The connection (`c=') and attribute (`a=') information in  the  session-
level section applies to all the media of that session unless overridden
by connection information or an attribute of the same name in the  media
description.   For instance, in the example below, each media behaves as
if it were given a `recvonly' attribute.

An example SDP description is:

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        o=mhandley 2890844526 2890842807 IN IP4
        s=SDP Seminar
        i=A Seminar on the session description protocol
        u= (Mark Handley)
        c=IN IP4
        t=2873397496 2873404696
        m=audio 3456 RTP/AVP 0
        m=video 2232 RTP/AVP 31
        m=whiteboard 32416 udp wb

Text records such as the session name and information  may  contain  any
printable  8  bit ISO 8859-1 character with the exceptions of 0x0a (new-
line) and 0x0d (carriage return).  Carriage Return  is  prohibited,  and
Newline is used to end a record.

Protocol Version


The ``v'' field gives the version of the Session  Description  Protocol.
There is no minor version number.


o=<username>  <session  id>  <version>  <network  type>  <address  type>

The ``o'' field gives the originator of the session (their username  and
the  address  of  the user's host) plus a session id and session version
number.  <username> is the user's login on the originating host,  or  it
is  ``-''  if  the originating host does not support the concept of user
ids.  <username> must not contain spaces.  <session  id>  is  a  numeric
string  such that the tuple of <username>, <session id>, <network type>,
<address type> and <address> form a globally unique identifier  for  the
session.   The  method  of  session  id allocation is up to the creating
tool, but it has been suggested  that  a  Network  Time  Protocol  (NTP)
timestamp  be  used  to  ensure  uniqueness [1].  <version> is a version
number for this announcement.  It is needed for proxy  announcements  to
detect  which  of several announcements for the same session is the most
recent.  Again its usage is up to the creating tool, so  long  as  <ver-
sion>  is  increased  when  a  modification is made to the session data.

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Again, it is recommended (but not mandatory) that an  NTP  timestamp  is
used.  <network type> is a text string giving the type of network.  Ini-
tially ``IN'' is defined to have  the  meaning  ``Internet''.   <address
type>  is  a  text  string  giving the type of the address that follows.
Initially ``IP4'' and ``IP6'' are defined.  <address>  is  the  globally
unique  address  of the machine from which the session was created.  For
an address type of IP4, this is the dotted-decimal representation of the
IP  version  4 address of the machine.  For an address type of IP6, this
is the compressed textual representation of the IP version 6 address  of
the machine.

Session Name

s=<session name>

The ``s'' field is the session name.  There must be  one  and  only  one
``s''  field  per announcement, and it must contain printable ISO 8859-1
characters (but see also the `charset' attribute below).

Session and Media Information

i=<session description>

The ``i'' field is information about the session.  There must be no more
than one session-level ``i'' field per session announcement. Although it
may be omitted, this is discouraged, and user interfaces  for  composing
sessions  should  require  text to be entered.  If it is present it must
contain printable ISO 8859-1 characters  (but  see  also  the  `charset'
attribute below).

A single ``i'' field can also be used for  each  media  definition.   In
media  definitions,  ``i''  fields  are  primarily intended for labeling
media streams.  As such, they are most likely to be useful when a single
session  has more than one distinct media stream of the same media type.
An example would be two different whiteboards, one for  slides  and  one
for feedback and questions.



o    A URI is a Universal Resource Identifier as used by WWW clients

o    The URI should be a pointer to  additional  information  about  the

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o    This field is optional, but if it is present it should be specified
    before the first media field

o    No more than one URI field is allowed per session description

Email Address and Phone Number

e=<email address>
p=<phone number>

o    These specify contact information for the  person  responsible  for
    the  conference.   This  is  not  necessarily  the  same person that
    created the conference announcement.

o    Either an email field or a phone field must  be  specified.   Addi-
    tional email and phone fields are allowed.

o    If these are present, they should be  specified  before  the  first
    media field.

o    More than one email or phone field  can  be  given  for  a  session

o    Phone numbers should be given  in  the  conventional  international
    format  -  preceded  by  a ``+'' and the international country code.
    There must be a space or a hyphen (``-'') between the  country  code
    and the rest of the phone number.  Spaces and hyphens may be used to
    split up a phone field to aid readability if desired. For example:

        p=+44-171-380-7777    or    p=+1 617 253 6011

o    Both email addresses and phone numbers can have  an  optional  free
    text  string  associated  with them, normally giving the name of the
    person who may be contacted.  This should be enclosed in parenthesis
    if it is present.  For example: (Mark Handley)

    The alternative RFC822 name quoting convention is also  allowed  for
    both email addresses and phone numbers.  For example,

        e=Mark Handley <>

    The free text string should be in the ISO 8859-1 character  set,  or
    alternatively  in  unicode UTF-7 encoding if the appropriate charset
    session-level attribute is set.

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Connection Data

c=<network type> <address type> <connection address>

The ``c'' field contains connection data.

The first sub-field is the network type, which is a text  string  giving
the  type  of  network.  Initially ``IN'' is defined to have the meaning

The second sub-field is the address type.  This allows SDP  to  be  used
for sessions that are not IP based.  Currently only IP4 is defined.

The third sub-field is the  connection  address.   Optional  extra  sub-
fields  may be added after the connection address depending on the value
of the <address type> field.

For IP4 addresses, the connection address is defined as follows:

o    Typically the connection address will be  a  class-D  IP  multicast
    group address.  If the conference is not multicast, then the connec-
    tion address contains the unicast IP address of  the  expected  data
    source or data relay or data sink as determined by additional attri-
    bute fields.  It is not expected  that  unicast  addresses  will  be
    given  in  a session description that is communicated by a multicast
    announcement, though this is not prohibited.

o    Conferences using an IP multicast connection address must also have
    a  time  to  live  (TTL)  value present in addition to the multicast
    address.  The TTL defines the scope  with  which  multicast  packets
    sent  in  this  conference should be sent. TTL values must be in the
    range 0-255.  The Mbone usage  guidelines  (currently  available  at  define  several  standard settings
    for TTL:

            local net:        1
            site:            15
            region:          63
            world:          127

    Other settings may have local meaning (e.g., 31 for all sites within
    an organization).

    The TTL for the session is appended to the address using a slash  as
    a separator.  An example is:

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            c=IN IP4

    Hierarchical or layered encoding schemes are data streams where  the
    encoding  from  a  single  media  source  is  split into a number of
    layers.  The receiver can choose  the  desired  quality  (and  hence
    bandwidth)  by  only  subscribing to a subset of these layers.  Such
    layered encodings are normally  transmitted  in  multiple  multicast
    groups  to  allow  multicast pruning.  This technique keeps unwanted
    traffic from sites only requiring certain levels of  the  hierarchy.
    For  applications  requiring multiple multicast groups, we allow the
    following notation to be used for the connection address:

            <base multicast address>/<ttl>/<number of addresses>

    If the number of addresses is not given it is  assumed  to  be  one.
    Multicast addresses so assigned are contiguously allocated above the
    base address, so that, for example:

            c=IN IP4

    would state that addresses, and are to
    be used at a ttl of 127.

    It is illegal for the slash notation described above to be used  for
    IP unicast addresses.

    A session announcement must contain one ``c'' field  in  each  media
    description  (see  below) or a ``c'' field at the session-level.  It
    may contain a session-level ``c'' field  and  one  additional  ``c''
    field  per  media  description,  in  which case the per-media values
    override the session-level settings for the relevant media.



o    This specifies the proposed bandwidth to be used by the session  or
    media, and is optional.

o    <bandwidth-value>  is in kilobits per second

o    <modifier>  is an single alphanumeric word giving  the  meaning  of
    the bandwidth figure.

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o    Two modifiers are initially defined:

CT    Conference Total: An implicit maximum bandwidth is associated with
      each TTL on the Mbone or within a particular multicast administra-
      tive scope region (the Mbone bandwidth vs. TTL limits are given in
      the  MBone FAQ).  If the bandwidth of a session or media in a ses-
      sion is different from the bandwidth implicit from  the  scope,  a
      `b=CT:...' line should be supplied for the session giving the pro-
      posed upper limit to the bandwidth used.  The primary  purpose  of
      this  is  to  give  an  approximate idea as to whether two or more
      conferences can co-exist simultaneously.

AS    Application Specific Maximum:  The bandwidth is interpreted to  be
      application  specific,  i.e., will be the application's concept of
      maximum bandwidth.  Normally this will coincide with what  is  set
      on the application's ``maximum bandwidth'' control if applicable.

    Note that CT gives a total bandwidth figure for all the media at all
    sites.   AS  gives a bandwidth figure for a single media at a single
    site, although there may be many sites sending simultaneously.

o    Extension Mechanism: Tool writers can define experimental bandwidth
    modifiers by prefixing their modifier with ``X-''.  For example:


    SDP parsers should ignore bandwidth fields with  unknown  modifiers.
    Modifiers  should  be alpha-numeric and, although no length limit is
    given, they are recommended to be short.

Times, Repeat Times and Time Zones

t=<start time>  <stop time>

o    ``t'' fields specify the start and stop times for a conference ses-
    sion.   Multiple  ``t'' fields may be used if a session is active at
    multiple irregularly  spaced  times;  each  additional  ``t''  field
    specifies an additional period of time for which the session will be
    active.  If the session is active at regular times,  an ``r''  field
    (see  below)  should  be  used  in addition to and following a ``t''
    field - in which case the  ``t'' field specifies the start and  stop
    times of the repeat sequence.

o    The first and second sub-fields give the start and stop  times  for
    the conference respectively.  These values are the decimal represen-
    tation of Network Time Protocol (NTP) time values  in  seconds  [1].
    To convert these values to UNIX time, subtract decimal 2208988800.

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o    If the stop-time is set to zero, then the session is  not  bounded,
    though it will not become active until after the start-time.  If the
    start-time is also zero, the session is regarded as permanent.

    User interfaces should strongly discourage the creation of unbounded
    and  permanent  sessions  as they give no information about when the
    session is actually going to terminate, and so make scheduling  dif-

    The general assumption may be made, when displaying  unbounded  ses-
    sions that have not timed out to the user, that an unbounded session
    will only be active until half an hour from the current time or  the
    session start time, whichever is the later.  If behaviour other than
    this is required, an  end-time  should  be  given  and  modified  as
    appropriate  when  new  information becomes available about when the
    session should really end.

    Permanent sessions may be shown to the user as  never  being  active
    unless  there are associated repeat times which state precisely when
    the session will be active.  In general, permanent  sessions  should
    not  be  created for any session expected to have a duration of less
    than 2 months, and should be discouraged for  sessions  expected  to
    have a duration of less than 6 months.

r=<repeat interval> <active duration> <list of offsets from start-time>

o     ``r'' fields specify repeat times for a session.  For example,  if
    a  session  is  active at 10am on Monday and 11am on Tuesday for one
    hour each week for three  months,  then  the  <start  time>  in  the
    corresponding ``t'' field would be the NTP representation of 10am on
    the first Monday, the <repeat interval> would be 1 week, the <active
    duration>  would  be  1  hour,  and the offsets would be zero and 25
    hours. The corresponding ``t'' field stop  time  would  be  the  NTP
    representation of the end of the last session three months later. By
    default all fields are in seconds, so the  ``r''  and  ``t''  fields
    might be:

        t=3034423619 3042462419
        r=604800 3600 0 90000

     To make announcements more compact, times  may  also  be  given  in
    units  of  days, hours or minutes.  The syntax for these is a number
    immediately followed by a single  case-sensitive  character.   Frac-
    tional  units  are  not  allowed  -  a  smaller  unit should be used
    instead.  The following unit specification characters are allowed:

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            d - days (86400 seconds)
            h - minutes (3600 seconds)
            m - minutes (60 seconds)
            s - seconds (allowed for completeness but not recommended)

    Thus, the above announcement could also have been written:

        r=7d 1h 0 25h

    Monthly and yearly repeats cannot currently  be  directly  specified
    with  a  single SDP repeat time - instead separate "t" fields should
    be used to explicitly list the session times.

z=<adjustment time> <offset> <adjustment time> <offset> ....

o    To schedule a repeated session which spans a change from  daylight-
    saving  time  to  standard  time  or  vice-versa, it is necessary to
    specify offsets from the base repeat times. This is required because
    different  time  zones  change  time at different times of day, dif-
    ferent countries change to or from daylight time on different dates,
    and some countries do not have daylight saving time at all.

    Thus in order to schedule a session that is at the same time  winter
    and  summer,  it must  be possible to specify unambiguously by whose
    time zone a  session  is  scheduled.   To  simplify  this  task  for
    receivers,  we  allow the sender to specify the NTP time that a time
    zone adjustment happens and the offset from the time when  the  ses-
    sion  was  first  scheduled.  The  ``z''  field allows the sender to
    specify a list of these adjustment times and offsets from  the  base

    An example might be:

    z=2882844526 -1h 2898848070 0

    This specifies that at time 2882844526 the time base  by  which  the
    session's repeat times are calculated is shifted back by 1 hour, and
    that  at  time  2898848070  the  session's  original  time  base  is
    restored.   Adjustments  are  always relative to the specified start
    time - they are not cumulative.

o    If a session is likely to last several years, it is  expected  that
    the  session  announcement will be modified periodically rather than
    transmit several years worth of adjustments in one announcement.

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Encryption Keys

k=<method>:<encryption key>

o    The session description protocol may be used to  convey  encryption
    keys.   A  key  field  is permitted before the first media entry (in
    which case it applies to all media in  the  session),  or  for  each
    media entry as required.

4   The format of keys and their usage is  outside  the  scope  of  this
    document, but see [3].

o    The method indicates the mechanism to be used to  obtain  a  usable
    key  by  external  means,  or from the encoded encryption key given.
    The following methods are defined:

    k=clear:<encryption key>
        The encryption key (as described in [3] for  RTP  media  streams
        under  the  AV  profile)  is  included untransformed in this key

    k=base64:<encoded encryption key>
        The encryption key (as described in [3] for  RTP  media  streams
        under the AV profile) is included in this key field but has been
        base64 encoded because it includes characters  that  are  prohi-
        bited in SDP.

    k=uri:<URI to obtain key>
        A Universal Resource  Identifier  as  used  by  WWW  clients  is
        included in this key field.  The URI refers to the data contain-
        ing the key, and may require  additional  authentication  before
        the  key  can  be returned.  When a request is made to the given
        URI, the MIME content-type of the reply specifies  the  encoding
        for  the key in the reply.  The key should not be obtained until
        the user wishes to join the session to reduce synchronisation of
        requests to the WWW server(s).

        No key is included in this SDP description, but the  session  or
        media  stream  referred  to by this key field is encrypted.  The
        user should be prompted for the key when attempting to join  the
        session,  and this user-supplied key should then used to decrypt
        the media streams.

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A media field may also have any  number  of  attributes  (``a''  fields)
which are media specific.  Attribute fields may be of two forms:

o    property attributes.  A property attribute is simply  of  the  form
    ``a=<flag>''.   These are binary attributes, and the presence of the
    attribute conveys that the attribute is a property of  the  session.
    An example might be ``a=recvonly''.

 o    value  attributes.    A   value   attribute   is   of   the   form
    ``a=<attribute>:<value>''.   An  example  might be that a whiteboard
    could have the value attribute ``a=orient:landscape''

Attribute interpretation depends on the media tool being invoked.   Thus
receivers  of  session  descriptions  should  be  configurable  in their
interpretation of announcements in general and of attributes in particu-

Attribute fields (``a'' fields) can also be added before the first media
field.   These  attributes  would  convey  additional  information  that
applies to the conference as a whole rather than  to  individual  media.
An example might be the conference's floor control policy.

Media Announcements

m=<media>  <port>  <transport> <fmt list>

A session announcement may contain  a  number  of  media  announcements.
Each media announcement starts with an ``m'' field, and is terminated by
either the next ``m'' field or by the end of the  session  announcement.
A media field also has several sub-fields:

o    The first sub-field is the media type.  Currently defined media are
    ``audio'',  ``video'', ``whiteboard'', ``text'' and ``data'', though
    this list may be extended as  new  communication  modalities  emerge
    (e.g., telepresence or conference control).

o    The second sub-field is the  transport  port  to  which  the  media
    stream  will  be sent.  The meaning of the transport port depends on
    the network being used as specified in the relevant ``c'' field  and
    on  the  transport  protocol  defined in the third sub-field.  Other
    ports used by the media application (such as the RTCP port, see [2])

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    should be derived algorithmically from the base media port.

    Note: For transports based on UDP, the value should be in the  range
    1024  to  65535  inclusive.  For RTP compliance it should be an even
    number.  If the port is allocated randomly by the creating  applica-
    tion, it is recommended that ports above 5000 are chosen as, on Unix
    systems, ports below 5000 may  be  allocated  automatically  by  the
    operating system.

    For applications where hierarchically encoded streams are being sent
    to  a unicast address, it may be necessary to specify multiple tran-
    sport ports.  This is done using a similar notation to that used for
    IP multicast addresses in the ``c'' field:

            m=<media> <port>/<number of ports> <transport> <fmt list>

    In such a case, the ports used depend  on  the  transport  protocol.
    For RTP, only the even ports are used for data and the corresponding
    one-higher odd port is used for RTCP.  For example:

            m=video 3456/2 RTP/AVP 31

    would specify that ports 3456 and 3457 form one  RTP/RTCP  pair  and
    3458  and  3459 form the second RTP/RTCP pair.  RTP/AVP is the tran-
    sport protocol and 31 is the format (see below).

    It is illegal for both multiple addresses to  be  specified  in  the
    ``c''  field  and  for  multiple  ports to be specified in the ``m''
    field in the same session announcement.

o    The third sub-field is the transport protocol.  The transport  pro-
    tocol  values  are  dependent on the address-type field in the ``c''
    fields.  Thus a ``c'' field of IP4 defines that the transport proto-
    col runs over IP4.  For IP4, it is normally expected that most media
    traffic will be carried as RTP over UDP.   The  following  transport
    protocols  are  preliminarily  defined,  but may be extended through
    registration of new protocols with IANA:

    - RTP/AVP  -  the  IETF's  Realtime  Transport  Protocol  using  the
      Audio/Video profile carried over UDP.

    - udp  - User Datagram Protocol

    If an application uses a single combined  proprietary  media  format
    and   transport  protocol  over  UDP,  then  simply  specifying  the

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    transport protocol as udp and using the format field to  distinguish
    the  combined  protocol  is recommended.  If a transport protocol is
    used over UDP to carry several distinct media types that need to  be
    distinguished  by a session directory, then specifying the transport
    protocol and media format separately is necessary.  RTP is an  exam-
    ple  of  a  transport  protocols that carry multiple payload formats
    that must be distinguished by the session directory for it  to  know
    how to start appropriate tools, relays, mixers or recorders.

    The main reason to specify the transport protocol in addition to the
    media  format is that the same standard media formats may be carried
    over different transport protocols even when the network protocol is
    the  same - a historical example is vat PCM audio and RTP PCM audio.
    In addition, relays and monitoring tools that are transport protocol
    specific but format independent are possible.

    For RTP media streams operating under the  RTP  Audio/Video  Profile
    [3],  the  protocol field is ``RTP/AVP''.  Should other RTP profiles
    be defined in the future, their profiles will be  specified  in  the
    same way.  For example, the protocol field ``RTP/XYZ'' would specify
    RTP operating under a profile whose short name is ``XYZ''.

o    The fourth and subsequent sub-fields are media formats.  For  audio
    and video, these will normally be a media payload type as defined in
    the RTP Audio/Video Profile.

    When a list of payload formats is given, this implies  that  all  of
    these  formats  may  be  used in the session, but the first of these
    formats is the default format for the session.

    For media whose transport protocol is not  RTP  or  UDP  the  format
    field  is  protocol  specific.  Such formats should be defined in an
    additional specification document.

    For media whose transport protocol is RTP, SDP can be used  to  pro-
    vide  a  dynamic binding of media encoding to RTP payload type.  The
    payload names in the RTP AV Profile  do  not  specify  unique  audio
    encodings (in terms of clock rate and number of audio channels), and
    so they are not used directly in SDP format  fields.   Instead,  the
    payload  type number should be used to specify the format for static
    payload types and the payload  type  number  along  with  additional
    encoding  information  should be used for dynamically allocated pay-
    load types.

    An example of a static payload type is u-law PCM coded single  chan-
    nel  audio  sampled  at 8KHz.  This is completely defined in the RTP
    Audio/Video profile as payload type 0, so the media field for such a
    stream sent to UDP port 3456 is:

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            m=video 3456 RTP/AVP 0

    An example of a dynamic payload type is 16 bit linear encoded stereo
    audio  sampled  at 16KHz.  If we wish to use dynamic RTP/AVP payload
    type 98 for such a stream, additional  information  is  required  to
    decode it:

            m=video 3456 RTP/AVP 98
            a=rtpmap:98 L16/16000/2

    The general form of an rtpmap attribute is:

            a=rtpmap:<payload type> <encoding name>/<clock rate>[/<encoding parameters>]

    For audio streams, <encoding parameters> may specify the  number  of
    audio  channels.   This  parameter  may  be omitted if the number of
    channels is one provided no additional parameters are needed.
    For video streams, no encoding parameters are currently specified.

    Additional parameters may be  defined  in  the  future,  but  codec-
    specific  parameters  should  not  be added.  Parameters added to an
    rtpmap attribute should only be those required for a session  direc-
    tory to make the choice of appropriate media too to participate in a
    session.  Codec-specific parameters should be added in other  attri-

    Up to one rtpmap attribute can  be  define  for  each  media  format
    specified.  Thus we might have:

            m=audio 12345 RTP/AVP 96 97 98
            a=rtpmap:96 L8/8000
            a=rtpmap:97 L16/8000
            a=rtpmap:98 L16/11025/2

    Experimental encoding formats can also be  specified  in  this  way.
    RTP  formats  that  are  not registered with IANA as standard format
    names must be preceded by ``X-''.  Thus a new experimental redundant
    audio  stream  called  GSMLPC using dynamic payload type 99 could be
    specified as:

            m=video 3456 RTP/AVP 99
            a=rtpmap:99 X-GSMLPC/8000

    Such an experimental encoding requires  that  any  site  wishing  to
    receive  the  media stream has relevant configured state in its ses-
    sion directory to know which tools are appropriate.

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    Note that RTP audio formats typically  do  not  include  information
    about  the  number  of  samples  per  packet.   If a non-default (as
    defined in the RTP Audio/Video Profile) packetisation  is  required,
    the``ptime'' attribute is used as given below.

    For more details on RTP audio and video formats, see [3].

o    Predefined formats for UDP protocol non-RTP media are as below.

    Whiteboard Formats:

      wb:   LBL Whiteboard (transport: udp)

    Text Formats:

      nt:   UCL Network Text Editor (transport: udp)

Suggested Attributes

The following attributes are suggested.  Since application  writers  may
add new attributes as they are required, this list is not exhaustive.

    This attribute gives the dot-separated hierarchical category of  the
    session.   This  is to enable a receiver to filter unwanted sessions
    by category.  It would probably  have  been  a  compulsory  separate
    field,  except  for  its  experimental nature at this time.  It is a
    session-level attribute.

    Like the cat attribute, this is to assist  identifying  wanted  ses-
    sions at the receiver.  This allows a receiver to select interesting
    session based on keywords describing the purpose of the session.  It
    is a session-level attribute.

a=tool:<name and version of tool>
    This gives the name and version number of the tool  used  to  create
    the session description.  It is a session-level attribute.

a=ptime:<packet time>
    This gives the length of time in  milliseconds  represented  by  the
    media  in a packet. This is probably only meaningful for audio data.
    It should not be necessary to know ptime to decode RTP or vat audio,

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    and    it    is    intended    as    a    recommendation   for   the
    encoding/packetisation of audio.  It is a media attribute.

    This specifies that the tools should be started in receive-only mode
    where applicable. It can be either a session or media attribute.

    This specifies that the tools should be started in send and  receive
    mode.  This is necessary for interactive conferences with tools such
    as wb which defaults to receive only mode. It can be either  a  ses-
    sion or media attribute.

    This specifies that the tools should be started in  send-only  mode.
    An  example  may  be where a different unicast address is to be used
    for a traffic destination than for a traffic source. In such a case,
    two media descriptions may be use, one sendonly and one recvonly. It
    can be either a session or media attribute, but would normally  only
    be used as a media attribute.

a=orient:<whiteboard orientation>
    Normally this is only used in a whiteboard media  specification.  It
    specifies  the orientation of a the whiteboard on the screen.  It is
    a media attribute.  Permitted values are `portrait', `landscape' and
    `seascape' (upside down landscape).

a=type:<conference type>
    This specifies the type of the  conference.   Suggested  values  are
    `broadcast',  `meeting', `moderated', `test' and `H332'.  `recvonly'
    should be the default for `type:broadcast' sessions,  `type:meeting'
    should imply `sendrecv' and `type:moderated' should indicate the use
    of a floor control tool and that the media tools are started  so  as
    to ``mute'' new sites joining the conference.

    Specifying the attribute type:H332 indicates that this loosely  cou-
    pled  session is part of a H.332 session as defined in the ITU H.332
    specification.  Media tools should be started `recvonly'.

    Specifying the attribute type:test is  suggested  as  a  hint  that,
    unless  explicitly  requested  otherwise, receivers can safely avoid
    displaying this session description to users.

    type is a session-level attribute.

a=charset:<character set>
    This specifies the character set to be used to display  the  session

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    name and information data.  By default, the ISO 8859-1 character set
    is used.  If the ISO 8859-1 character set is not suitable,  the  use
    of  unicode (ISO 10646) [6],[7], as specified in RFC1641 [8] is sug-
    gested.  In particular, the UTF-7 (RFC1642)  [9]  encoding  is  sug-
    gested with the following SDP attribute:


    This is a session-level attribute; if this attribute is present,  it
    must be before the first media field.

a=framerate:<frame rate>
    This gives the maximum  video  frame  rate  in  frames/sec.   It  is
    intended  as  a  recommendation  for  the  encoding  of  video data.
    Decimal representations of  fractional  values  using  the  notation
    "<integer>.<fraction>" are allowed.  It is a media attribute, and is
    only defined for video media.

    This gives a suggestion for  the  quality  of  the  encoding  as  an
    integer value.

    The intention of the quality attribute for video  is  to  specify  a
    non-default  trade-off  between  frame-rate and still-image quality.
    For video, the value in the range 0 to 10, with the  following  sug-
    gested meaning:

    10  - the best still-image quality the compression scheme can give.

    5   - the default behaviour given no quality suggestion.

    0   - the worst still-image quality the  codec  designer  thinks  is
        still usable.

a=fmtp:<format> <format specific parameters>
    This attribute allows parameters that are specific to  a  particular
    format  to  be conveyed in a way that SDP doesn't have to understand
    them.  The format must be one  of  the  formats  specified  for  the
    media.   Format-specific  parameters  may  be  any set of parameters
    required to be conveyed by SDP and given unchanged a the media  tool
    that will use this format.

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6.1.  Communicating Conference Control Policy

There is some debate over the way conference control  policy  should  be
communicated.  In general, the authors believe that an implicit declara-
tive style of specifying conference control is desirable where possible.

A simple declarative style uses  a  single  conference  attribute  field
before  the  first media field, possibly supplemented by properties such
as `recvonly' for some of the media tools.   This  conference  attribute
conveys the conference control policy.  An example might be:


In some cases, however, it is possible that this may be insufficient  to
communicate  the  details  of  an unusual conference control policy.  If
this is the case, then a conference attribute specifying  external  con-
trol  might  be set, and then one or more ``media'' fields might be used
to specify the conference control tools and configuration data for those
tools.  An example is an ITU H.332 session:

                c=IN IP4
                m=audio 12345 RTP/AVP 0
                m=video 12347 RTP/AVP 31
                m=whiteboard 12349 udp wb
                m=control 12341 H323 mc
                c=IN IP4

In this example, a general conference attribute (type:H332) is specified
stating  that  conference  control will be provided by an external H.332
tool, and a contact addresses for  the  H.323  session  multipoint  con-
troller is given.

In this document, only  the  declaritive  style  of  conference  control
declaration  is  specified.   Other  forms  of conference control should
specify an appropriate type attribute, and should  define  the  implica-
tions this has for control media.

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Appendix A: SDP Grammar

announcement ::=        proto-version

proto-version ::=       "v=" (DIGIT)+
                        ;this draft describes version 0

origin-field ::=        "o=" username space
                        sess-id space sess-version space
                        nettype space addrtype space
                        addr newline

session-name-field ::=  "s=" text

information-field ::=   ["i=" text newline]

uri-field ::=           ["u=" uri newline]

email-fields ::=        ("e=" email-address newline)*

phone-fields ::=        ("p=" phone-number newline)*

connection-field ::=    ["c=" nettype space addrtype space
                        connection-address newline]
                        ;a connection field must be present
                        ;in every media description or at the

bandwidth-fields ::=    ("b=" bwtype ":" bandwidth newline)*

time-fields ::=         ( "t=" start-time space stop-time
                          (newline repeat-fields)* newline)+
                        [zone-adjustments newline]

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repeat-fields ::=       "r=" repeat-interval space typed-time
                        (space typed-time)+

zone-adjustments ::=    time space [``-''] typed-time
                        (space time space [``-''] typed-time)*

key-field ::=           ["k=" key-type newline]

key-type ::=            "prompt" |
                        "clear:" key-data |
                        "base64:" key-data |
                        "uri:" uri

key-data ::=            printable-ascii

attribute-fields ::=    ("a=" attribute newline)*

media-descriptions ::=  ( media-field
                          attribute-fields )*

media-field ::=         "m=" media space port ["/" integer]
                         space proto (space fmt)+ newline

media ::=               (alpha-numeric)+
                        ;typically "audio", "video", "whiteboard"
                        ;or "text"

fmt ::=                 (alpha-numeric)+
                        ;typically an RTP payload type for audio
                        ;and video media

proto ::=               (alpha-numeric)+
                        ;typically "RTP/AVP" or "udp" for IP4

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port ::=                (DIGIT)+
                        ;should in the range "1024" to "65535" inclusive
                        ;for UDP based media ;random allocation should
                        ;only assign above UDP port "5000".

attribute ::=           att-field ":" att-value | att-field

att-field ::=           (ALPHA)+

att-value ::=           (att-char)+

att-char ::=            alpha-numeric | "-"
                        ;is this too tight a restriction

sess-id ::=             (DIGIT)+
                        ;should be unique for this originating username/host

sess-version ::=        (DIGIT)+
                        ;0 is a new session

connection-address ::=  multicast-conf-address | multicast-scoped-address
                        | unicast-address

multicast-conf-address ::=
                        "224.2." decimal_uchar "." decimal_uchar "/" ttl
                        [ "/" integer ]
                        ;multicast addresses may be in a larger range
                        ;but only these should be assigned by an sdp tool

multicast-scoped-address ::=
                        "239."  decimal_uchar "." decimal_uchar "."
                        decimal_uchar "/" ttl [ "/" integer ]

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ttl ::=                 decimal_uchar

start-time ::=          time | "0"

stop-time ::=           time | "0"

time ::=                POS-DIGIT 9*DIGIT
                        ;sufficient for 2 more centuries

repeat-interval ::=     typed-time | interval-time

typed-time ::=          (DIGIT)+ [fixed-len-time-unit]

interval-time ::=       (DIGIT)+ variable-len-time-unit

fixed-len-time-unit ::= ``d'' | ``h'' | ``m'' | ``s''

variable-len-time-unit ::= ``Y'' | ``M''

bwtype ::=              (alpha-numeric)+

bandwidth ::=           (DIGIT)+

username ::=            safe
                        ;pretty wide definition, but doesn't include space

email-address ::=       email | email "(" email-safe ")" |
                        email-safe "<" email ">"

email ::=               ;defined in RFC822

uri::=                  ;defined in RFC1630

phone-number ::=        phone | phone "(" email-safe ")" |
                        email-safe "<" phone ">"

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phone ::=               "+" POS-DIGIT (space | "-" | DIGIT)+
                        ;there must be a space or hyphen between the
                        ;international code and the rest of the number.

nettype ::=             "IN"
                        ;list to be extended

addrtype ::=            "IP4" | "IP6"
                        ;list to be extended

addr ::=                unicast-address

unicast-address ::=     IP4-address | IP6-address

IP4-address ::=         b1 "." decimal_uchar "." decimal_uchar "." b4
b1 ::=                  decimal_uchar
                        ;less than "224"; not "0" or "127"
b4 ::=                  decimal_uchar
                        ;not "0"

IP6-address ::=         ;to be defined

text ::=                (printable-iso8859-1)+ | (unicode-1-1-utf-7)+
                        ;unicode requires a "a=charset:unicode-1-1-utf-7"
                        ;attribute to be used

printable-iso8859-1 ::= ;8 bit ascii character
                        ;decimal 9 (TAB), 32-126 and 161-255

unicode-1-1-utf-7 ::=   unicode-safe
                        ;defined in RFC 1642

decimal_uchar ::=       DIGIT
                        | POS-DIGIT DIGIT
                        | (1 2*DIGIT)
                        | (2 (0|1|2|3|4) DIGIT)
                        | (2 5 (0|1|2|3|4|5))

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integer ::= POS-DIGIT (DIGIT)*

alpha-numeric ::=       ALPHA | DIGIT

printable-ascii ::=     unicode-safe | "~" | "

DIGIT ::=               0 | POS-DIGIT

POS-DIGIT ::=           1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

ALPHA ::=               a | b | c | d | e | f | g | h | i | j | k |
                        l | m | n | o  | p | q | r | s | t | u | v |
                        w | x | y | z | A | B | C  | D | E | F | G |
                        H | I | J | K | L | M | N | O | P |  Q | R |
                        S | T | U | V | W | X | Y | Z

unicode-safe ::=        email-safe | "(" | ")" | "<" | ">"
                        ;although unicode allows newline and carriage
                        ;return, we don't here.

email-safe ::=          safe | space | tab

safe ::=                alpha-numeric |
                        "'" | "'" | "-" | "." | "/" | ":" | "?" | """ |
                        "#" | "$" | "&" | "*" | ";" | "=" | "@" | "[" |
                        "]" | "^" | "_" | "`" | "{" | "|" | "}" | "+" |
                        "~" | "

space ::=               ;ascii code 32
tab ::=                 ;ascii code 9
newline ::=             ;ascii code 10

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Appendix C: Authors' Addresses

Mark Handley
Information Sciences Institute
c/o MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139
United States
electronic mail:

Van Jacobson
MS 46a-1121
Lawrence Berkeley Laboratory
Berkeley, CA 94720
United States
electronic mail:


Many people in the IETF MMUSIC working  group  have  made  comments  and
suggestions contributing to this document.  In particular, we would like
to thank Eve Schooler, Steve Casner, Bill Fenner, Allison  Mankin,  Ross
Finlayson, Peter Parnes, Joerg Ott and Carsten Bormann.


[1] D. Mills, ``Network Time Protocol version 2 specification and imple-
mentation", RFC1119, 1st Sept 1989.

[2] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, ``RTP: A Tran-
sport Protocol for Real-Time Applications'', RFC 1889

[3] H. Schulzrinne, ``RTP Profile for Audio and Video  Conferences  with
Minimal Control'', RFC 1890

[4] M. Handley, ``SAP - Session Announcement Protocol'', INTERNET-DRAFT,
November 25th 1996.

[5] V. Jacobson, S. McCanne, ``vat -  X11-based  audio  teleconferencing
tool'' vat manual page, Lawrence Berkeley Laboratory, 1994.

[6] ``The Unicode Standard, Version 1.1'': Version 1.0, Volume  1  (ISBN
0-201-56788-1), Version 1.0, Volume 2 (ISBN 0-201-60845-6), and "Unicode
Technical Report #4, The Unicode Standard, Version 1.1" (available  from
The Unicode Consortium, and soon to be published by Addison- Wesley).

[7] ISO/IEC 10646-1:1993(E) Information Technology--Universal  Multiple-

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INTERNET-DRAFT                                           24th March 1997

octet Coded Character Set (UCS).

[8] D. Goldsmith, M. Davis, ``Using Unicode with MIME'',  RFC1641,  July

[9] D. Goldsmith, M. Davis, ``UTF-7 - A Mail-Safe Transformation  Format
of Unicode'', RFC1642, July 1994

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