Internet Engineering Task Force					  MMUSIC WG
INTERNET-DRAFT					  Mark Handley/Van Jacobson
draft-ietf-mmusic-sdp-03.txt					   ISI/LBNL
							      21st Nov 1996
							    26th March 1997
						    Expires: 21st May 26th Sept 1997

		   SDP:	Session	Description Protocol

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     This document defined 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.

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

This document defines a session description protocol  that  is	used  to
define multimedia sessions.  It  SDP is also an attempt to provide guidelines designed to the writers of convey such announcement tools information to  protect	the  Mbone  from
misuse recipients.
SDP  is	 purely	a format for session description - it does not incorpor-
tate a transport protocol, and is intended to preserve  use  different  transport
protocols  as  appropriate  including  the inherent scalability of Session Announcement	Protocol
[4], electronic	mail using the original LBNL sd
session directory program whilst enhancing its functionality.

We have also attempted to enhance MIME extensions,	and the generality of	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 session 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.

    A multimedia session is a set a of multimedia	 senders  and  receivers
    and	 the  data  streams  flowing  from senders to receivers.  A multimedia 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  is  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.	Normally
it	A common
mode  of  usage	 is	used  by  an  SDP  for  a  client that announces  to	announce a conference session by
periodically multicasting an announcement packet on to a well known  multi-
cast address and port.	With the advent of administrative scoping in the
Mbone, it is likely that session directory clients will need to be  able
to listen for such announcements on multiple addresses.	 This multicast-
ing of announcements is performed by 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

use of the MIME	content	type
``application/x-sdp''  is  currently  suggested. ``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,  nor	do  they
allow scalable dynamic  session  because  the
multicast address allocation.  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:


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:

+   Contact

o    Remote address for	media


o    Transport port for	contact	address

This may or may not be be

The semantics of this address and port depend on the	source media and  destination  of 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

Sessions being conveyed over other types of network will have their  own
specific requirements - SDP must be extensible may define to	use these to establish a
control	channel	for these. 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


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 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

SDP about a Session

A session description should convey enough information to decide whether
or not to  parti-
cipate participate in a session.  It should  SDP may include additional	pointers
in the form of Universal Resources Identifiers (URIs) for more information	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  should  support supports a
categorisation  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 should also allow allows extensions to allow other font types character	sets  to  be  used  when

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.

Each announcement

A session description consists of a session-level descriptions	(details
that  apply  to	 the whole session description 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'	 description  media-level  sections.   The session description session-level part starts with a `v='
line and continues to the first media description  or
the next session description.	media-level section.  The media description	descrip-
tion  starts  with an `m=' line	and continues to the next media description	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. 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	`*'.

	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	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:

	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 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 triple tuple of <username>, <session id> id>, <network type>,
<address type> and <address> form a globally unique identifier	for  the
session.   The	method	of  ses-
sion  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	 unique-
ness	 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 <version>  <ver-
sion>  is  increased  when  a  modification is made to the session data.

Again, it is recommended (but not mandatory) that an  NTP  timestamp  is
used.  <network	type> is a text	string giving the type of network.	 Initially  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 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


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 for-
    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 an IS0-8859-1 the ISO 8859-1 character	set,  or
    alternatively  in  unicode UTF-7 encoding if the appropriate charset
    session-level attribute is set.

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:

	    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.


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
      administrative 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 session ses-
      sion is different	from the bandwidth implicit from  the  scope,  a
      `b=CT:...' line should be	supplied for the session giving	the proposed pro-
      posed upper limit	to the bandwidth used.	The  pri-
      mary 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.


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.

+   It is prohibited for the start time to be after the stop time.

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 correspond-
    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:

	    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>	....

+   Should it be necessary to

o    To	schedule a repeated session which spans	a change from daylight-saving  daylight-
    saving  time  to  standard	time  or  vice-versa, it is necessary to
    specify offsets from the base repeat times.	This is
    necessary	required because
    different  time  zones  change  time at different times of day, different dif-
    ferent countries change to or from daylight	time on	different dates,
    and	some countries to 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.

Encryption Keys

k=<method>:<encryption key>

+   In countries where encrypted sessions are not prohibited by law, the

o    The session description protocol may be used to  convey  encryption

+   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.




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

+   flag

o    property attributes.  A  flag property attribute	is simply  of  the  form
    ``a=<flag>''.   These are binary attributes, and the presence of the
    attribute conveys that the attribute is ``true''. 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., telepresense 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 net-
    work	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])
    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 application,	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 proto-
    col  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.  However,  some  com-
    monly  used	 applications  such as vat [5] do not use RTP.	Thus the   The  following  transport
    protocols  are defined:  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.

    - VAT  - LBL's Visual Audio Tool packet format carried over UDP.

    - UDP udp  - User Datagram Protocol

    If an application uses a single combined propriety  proprietary  media  format
    and	  transport  protocol  over  UDP,  then	 simply	 specifying  the
    transport protocol as UDP udp and using	the format field to  distinguish
    the  com-
    bined	 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.	VAT and	RTP  are
    examples 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 -  for	a historical example is	vat PCM	audio and RTP PCM audio.
    In addi-
    tion, 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:

	    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> optionally specifies 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.

    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) udp)

    Text Formats:


      nt:   UCL	Network	Text Editor (transport: UDP)	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 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 receiver.	session	description.  It is a session 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,
    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.
    Typically  this
    An	example	 may  be used 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 `moderated'. `H332'.  `recvonly'
    should be the default for `type:broadcast' sessions,  `type:meeting'
    should imply `sendrecv' and	`type:moderated' should imply	indicate the use
    of a floor con-
    trol 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 also  suggested  as  a  hint  that,
    unless  explicitly	requested  otherwise, receivers	can safely avoid
    displaying this session description	to users.

    type is a session session-level attribute.

a=charset:<character set>
    This specifies the character set to	be used	to display  the	 session
    name and information data.	By default, an the	ISO 8859-1 character set
    is used.  If an 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 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.

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 flags properties such
as `recvonly' for some of the media tools.   This  conference  attribute  con-
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.	A fictional	An example might be: 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 udp wb
		m=control 12341 UDP SCCP
		a=whiteboard:chaired	H323 mc
		c=IN IP4

In this fictional example (i.e., this is not  implemented  anywhere),	example, a general conference attribute	(type:H332) is specified
stating	 that  conference  control will	be provided by an external H.332
tool, and  specific  attributes
are given to specify a contact addresses for  the control policy that tool should use.	H.323  session	multipoint  con-
troller	is given.

In this	document, only	the former  declaritive  style	 of  conference	 control	declara-
declaration  is  specified, though we recognise that some variant on	 specified.   Other  forms  of conference control should
specify	an appropriate type attribute, and should  define  the latter
may also be used eventually.	implica-
tions this has for control media.

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]

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", "VAT", "RTP/AVP" or "UDP"	"udp" for IP4

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 ]

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 ::=		alpha-numeric
			;perhaps this is too restrictive...		safe
			;pretty	wide definition, but doesn't include space

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

email ::=		;defined in RFC822

uri::=			;defined in RFC1630

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

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
			| (1 2*DIGIT)
			| (2 (0|1|2|3|4) DIGIT)
			| (2 5 (0|1|2|3|4|5))

integer	::= POS-DIGIT (DIGIT)*

alpha-numeric ::=	ALPHA |	DIGIT

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


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

email-safe ::=		safe | space | tab

safe ::=		alpha-numeric |
			"'" | "'" | "-"	| "." |	"/" | ":" | "?"	| """ |
			"#" | "$" | "&"	| "*" |	";" | "<" | "=" | ">" | "@"	| "[" |
			"]" | "^" | "_"	| "`" |	"{" | "|" | "}"	| "+" | space
			"~" | tab
			;although unicode allows newline and carriage
			;return, we don't here. "

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

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  and  Mankin,  Ross Finlayson.
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-

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