Internet DRAFT - draft-ietf-mmusic-sdp


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Internet Engineering Task Force					  MMUSIC WG
INTERNET-DRAFT					  Mark Handley/Van Jacobson					   ISI/LBNL
							      22nd Jan 1997
						     Expires: 22nd Jul 1997

		   SDP:	Session	Description Protocol

Status of this Memo

This document is an Internet-Draft.  Internet-Drafts are  working  docu-
ments  of the Internet Engineering Task	Force (IETF), its areas, and its
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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 multimedia 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 negotiation of	media encodings.

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 incorporate
a transport protocol, and is intended to use different transport  proto-
cols  as  appropriate  including  the Session Announcement Protocol [4],
Session	Initiation Protocol [11],  Real-Time  Streaming	 Protocol  [12],
electronic  mail  using	the MIME extensions, and the Hypertext Transport

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	which data is sent, and	the remote address  and	 local	port  on
which  to  receive 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 the	ISO 10646 character sets
in the UTF-8 encoding (RFC 2044) to allow many different languages to be
represented.  However, to assist in compact  representations,  SDP  also
allows	other character	sets such as ISO 8859-1	to be used when	desired.
Internationalization only applies to free-text fields (session name  and
background information), and not to SDP	as a whole.

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6.  SDP	Specification

SDP session descriptions are entirely textual using the	ISO 10646  char-
acter  set  in UTF-8 encoding.	SDP field names	and attributes names use
only the US-ASCII subset of UTF-8,  but	 textual  fields  and  attribute
values	may  use the full ISO 10646 character set.  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>.	 It also
will be	case-significant unless	 a  specific  field  defines  otherwise.
Whitespace  is	not  permitted	either	side of	the `='	sign. In general
<value>	is either a number of fields delimited by a single space charac-
ter or a free format string.

A session description consists of a session-level  description	(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

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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 (see below)
		z=* (time zone adjustments)
		k=* (encryption	key)
		a=* (zero or more session attribute lines)
		Zero or	more media descriptions	(see below)

	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  ("a=" 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  ses-
sion 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	49170 RTP/AVP 0
	m=video	51372 RTP/AVP 31
	m=application 32416 udp	wb

Text records such as the session name and information are bytes	 strings
which  may  contain  any  byte	with  the exceptions of	0x00 (Nul), 0x0a
(ASCII newline)	and 0x0d (ASCII	carriage  return).   The  sequence  CRLF
(0x0d0a)  is  used  to end a record, although parsers should be	tolerant
and also accept	records	terminated with	a single newline character.   By
default	 these byte strings contain ISO-10646 characters in UTF-8 encod-
ing, but this default may be changed using the `charset' attribute.

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

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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.
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.

In general, the	``o='' field serves as a globally unique identifier  for
this  version  of  this	session	description, and the subfields excepting
the version taken together identify  the  session  irrespective	 of  any

Session	Name

s=<session name>

The ``s='' field is the	session	name.  There must be one  and  only  one
``s=''	field  per  session  description,  and it must contain ISO 10646
characters (but	see also the `charset' attribute below).

Session	and Media Information

i=<session description>

The ``i='' field is information	about the session.  There may be at most
one  session-level ``i='' field	per session description, and at	most one
``i='' field per media.	Although it may	be omitted, this is  discouraged
for  session  announcements,  and user interfaces for composing	sessions
should require text to be entered.  If it is present it	must contain ISO
10646 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.


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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
    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)

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    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-10646 character  set  with
    UTF-8 encoding, or alternatively in	ISO-8859-1 or other encodings if
    the	appropriate charset session-level attribute is set.

Connection Data

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

The ``c='' field contains connection data.

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.

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 and the address together	define	the  scope  with
    which  multicast  packets  sent in this conference will be sent. TTL
    values must	be in the range	0-255.

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    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.  This is semantically identical to includ-
    ing	multiple ``c=''	lines in a media description:

	    c=IN IP4
	    c=IN IP4
	    c=IN IP4

    Multiple addresses or ``c='' lines can only	be specified on	 a  per-
    media basis, and not for a session-level ``c='' field.

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



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

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o    <modifier>	 is a 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 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
    session.   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

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    representation of Network Time Protocol (NTP) time values in seconds
    [1].   To  convert	these  values  to  UNIX	 time,	subtract decimal

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.

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    Fractional	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>	....

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.

o    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 be used to
	decrypt	the media streams.

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Attributes are the primary means for extending SDP.  Attributes	 may  be
defined	 to  be	used as	"session-level"	attributes, "media-level" attri-
butes, or both.

A media	description may	have any number	of  attributes	(``a=''	 fields)
which  are media specific.  These are refered to as "media-level" attri-
butes and add information about	the media stream.  Attribute fields  can
also be	added before the first media field; these "session-level" attri-
butes 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.

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 names	must be	in the US-ASCII	subset of ISO-10646/UTF-8.

Attribute values are byte strings, and MAY use	any  byte  value  except
0x00  (Nul), 0x0A (LF),	and 0x0D (CR).	By default, attribute values are
to be interpreted as in	ISO-10646 character  set  with	UTF-8  encoding.
Unlike	other text fields, attribute values are	NOT normally affected by
the `charset' attribute	as this	would  make  comparisons  against  known
values	problematic.   However,	 when an attribute is defined, it can be
defined	to be charset-dependent, in which  case	 it's  value  should  be
interpreted in the session charset rather than in ISO-10646.

Attributes that	will be	commonly used can be registered	with  IANA  (see
Appendix  B).  Unregistered attributes should begin with "X-" to prevent
inadvertant collision with registered attributes.  In either case, if an
attribute  is  received	 that  is  not	understood,  it	should simply be

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ignored	by the receiver.

Media Announcements

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

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

o    The first sub-field is the	media type.  Currently defined media are
    ``audio'',	``video'',  ``application'',  ``data''	and ``control'',
    though this	list may be extended  as  new  communication  modalities
    emerge (e.g., telepresense).  The difference between ``application''
    and	``data'' is that the former is a media flow such  as  whiteboard
    information, and the latter	is bulk-data transfer such as multicast-
    ing	of program executables which will not typically	be displayed  to
    the	 user.	 ``control'' is	used to	specify	an additional conference
    control channel for	the session.

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])
    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

    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 49170/2 RTP/AVP 31

    would specify that ports 49170 and 49171 form one RTP/RTCP pair  and

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    49172 and 49173 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 description.

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  pro-
    tocol  runs	 over  IP4.   For IP4, it is normally expected that most
    media traffic will be carried as RTP over UDP.  The	following  tran-
    sport  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 tran-
    sport 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  dis-
    tinguished	by  a  session	directory, then	specifying the transport
    protocol and media format separately is necessary.	RTP is an  exam-
    ple	 of  a	transport-protocol that	carries	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

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    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
    encoding 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 49232 is:

	    m=video 49232 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 49232 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

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    directory to make the choice of appropriate	media too to participate
    in a session.  Codec-specific parameters should be	added  in  other

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

	    m=audio 49230 RTP/AVP 96 97	98
	    a=rtpmap:96	L8/8000
	    a=rtpmap:97	L16/8000
	    a=rtpmap:98	L16/11025/2

    RTP	profiles that specify the use  of  dynamic  payload  types  must
    define  the	 set  of valid encoding	names and/or a means to	register
    encoding names if that profile is to be used with SDP.

    Experimental encoding formats can also be  specified  using	 rtpmap.
    RTP	formats	that are not registered	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 49232 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.

    Application	Formats:

      wb:   LBL	Whiteboard (transport: udp)

      nt:   UCL	Network	Text Editor (transport:	udp)

Suggested Attributes

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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, and is not	dependent on charset.

    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. It is a charset dependent attribute,
    meaning that its value should be interpreted in the	 charset  speci-
    fied  for the session description if one is	specified, or by default
    in ISO 10646/UTF-8.

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, and is
    not	dependent on charset.

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,	 and  is
    not	dependent on charset.

    This specifies that	the tools should be started in receive-only mode
    where applicable. It can be	either a session or media attribute, and
    is not dependent on	charset.

    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, and is not	dependent on charset.

    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,

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    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, and is not dependent on charset.

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).	It is not dependent on charset

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 [10].	 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.

    The	type attribute is a session-level attribute, and is  not  depen-
    dent on charset.

a=charset:<character set>
    This specifies the character set to	be used	to display  the	 session
    name  and information data.	 By default, the ISO-10646 character set
    in UTF-8 encoding is used.	If  a  more  compact  representation  is
    required,  other  character	 sets may be used such as ISO-8859-1 for
    Northern European languages.   In  particular,  the	 ISO  8859-1  is
    specified with the following SDP attribute:


    This is a session-level attribute; if this attribute is present,  it
    must be before the first media field.  The charset specified MUST be
    one	of those registered with IANA, such as ISO-8859-1.  The	 charac-
    ter	set identifier is a US-ASCII string and	MUST be	compared against
    the	IANA identifiers using a case-insensitive  comparison.	 If  the
    identifier	is not recognised or not supported, all	strings	that are
    affected by	it SHOULD be regarded as byte strings.

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    Note that a	character set specified	MUST still prohibit the	 use  of
    bytes 0x00 (Nul), 0x0A (LF)	and 0x0d (CR).	Character sets requiring
    the	use of these characters	MUST define  a	quoting	 mechanism  that
    prevents these bytes appearing within text fields.

a=sdplang:<language tag>
    This can be	a session level	attribute or a	media  level  attribute.
    As a session level attribute, it specifies the language for	the ses-
    sion description.  As a media  level  attribute,  it  specifies  the
    language  for  any media-level SDP information field associated with
    that media.	 Multiple sdplang attributes can be provided  either  at
    session or media level if multiple languages if the	session	descrip-
    tion or media use multiple languages, in which case	the order of the
    attributes	 indicates  the	 order	of  importance	of  the	 various
    languages in the session or	 media	from  most  important  to  least

    The	sdplang	attribute value	must be	a single RFC 1766  language  tag
    in	US-ASCII.   It	is  not	 dependent on the charset attribute.  An
    sdplang attribute SHOULD be	specified when a session  is  of  suffi-
    cient  scope  to  cross  geographic	boundaries where the language of
    recipients cannot be assumed, or where the session is in a different
    language from the locally assumed norm.

a=lang:<language tag>
    This can be	a session level	attribute or a	media  level  attribute.
    As	a session level	attribute, it specifies	the default language for
    the	session	being described.  As a media level attribute, it  speci-
    fies  the  language	 for  that  media,  overriding any session-level
    language specified.	 Multiple lang attributes can be provided either
    at	session	 or  media  level  if  multiple	languages if the session
    description	or media use multiple languages, in which case the order
    of	the  attributes	indicates the order of importance of the various
    languages in the session or	 media	from  most  important  to  least

    The	lang attribute value must be a single RFC 1766 language	 tag  in
    US-ASCII.	It  is	not  dependent on the charset attribute.  A lang
    attribute SHOULD be	specified when a session is of sufficient  scope
    to cross geographic	boundaries where the language of recipients can-
    not	be assumed, or where the session is in a different language from
    the	locally	assumed	norm.

a=framerate:<frame rate>
    This gives the maximum  video  frame  rate	in  frames/sec.	  It  is

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    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, is
    only defined for video media, and is not dependent on charset.

    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.

    It is a media attribute, and is not	dependent on charset.

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 to the media tool
    that will use this format.

    It is a media attribute, and is not	dependent on charset.

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:


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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	49230 RTP/AVP 0
		m=video	49232 RTP/AVP 31
		m=application 12349 udp	wb
		m=control 49234	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  declarative  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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7.  Security Considerations

SDP is a session description format that describes multimedia  sessions.
A  session description should not be trusted unless it has been	obtained
by an authenticated transport protocol from a trusted source.  Many dif-
ferent	transport  protocols  may be used to distribute	session	descrip-
tion, and the nature of	the authentication will	differ from transport to

One transport  that  will  frequently  be  used	 to  distribute	 session
descriptions  is  the Session Announcement Protocol (SAP).  SAP	provides
both encryption	and authentication mechanisms but due to the  nature  of
session	 announcements	it is likely that there	are many occasions where
the originator of a session announcement cannot	be authenticated because
they  are  previously  unknown	to  the	receiver of the	announcement and
because	no common public key infrastructure is available.

On receiving a session description  over  an  unauthenticated  transport
mechanism  or  from  an	 untrusted  party,  software parsing the session
should take a few precautions.	Session	description contain  information
required  to  start  software  on  the	receivers system.  Software that
parses a session description MUST not be able to  start	 other	software
except	that which is specifically configured as appropriate software to
participate in multimedia sessions.  It	 is  normally  considered  INAP-
PROPRIATE  for	software  parsing  a  session description to start, on a
user's system, software	that is	appropriate to participate in multimedia
sessions,  without the user first being	informed that such software will
be started and giving their consent.  Thus a session description  arriv-
ing  by	 session  announcement,	 email,	 session invitation, or	WWW page
SHOULD not deliver the user into an {it	interactive} multimedia	 session
without	the user being aware that this will happen.  As	it is not always
simple to tell whether a session is  interactive  or  not,  applications
that are unsure	should assume sessions are interactive.

In this	specification, there are no attributes	which  would  allow  the
recipient  of  a  session description to be informed to	start multimedia
tools in a mode	where they default to  transmitting.   Under  some  cir-
cumstances  it	might be appropriate to	define such attributes.	 If this
is done	an application parsing a  session  description	containing  such
attributes  SHOULD  either  ignore them, or inform the user that joining
this session will result in the	 automatic  transmission  of  multimedia
data.  The default behaviour for an unknown attribute is to ignore it.

Session	descriptions may be  parsed  at	 intermediate  systems	such  as
firewalls  for	the  purposes of opening a hole	in the firewall	to allow
the participation in multimedia	sessions.  It is considered  INAPPROPRI-
ATE  for  a  firewall to open such holes for unicast data streams unless
the session description	comes in a request  from  inside  the  firewall.

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For  multicast	sessions,  it  is  likely that local administrators will
apply their own	policies, but the exclusive use	 of  "local"  or  "site-
local"	administrative	scope within the firewall and the refusal of the
firewall to open a hole	for such scopes	will provide separation	of  glo-
bal multicast sessions from local ones.

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Appendix A: SDP	Grammar

This appendix provides an Augmented BNF	grammar	for SDP.

announcement ::=	proto-version

proto-version ::=	"v=" 1*(DIGIT) CRLF
			;this draft describes version 0

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

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

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

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

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

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

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

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

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time-fields ::=		1*( "t=" start-time space stop-time
			  *(CRLF repeat-fields)	CRLF)
			[zone-adjustments CRLF]

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

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

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

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

key-data ::=		email-safe | "~" | "

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

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

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

media ::=		1*(alpha-numeric)
			;typically "audio", "video", "application"
			;or "data"

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fmt ::=			1*(alpha-numeric)
			;typically an RTP payload type for audio
			;and video media

proto ::=		1*(alpha-numeric)
			;typically "RTP/AVP" or	"udp" for IP4

port ::=		1*(DIGIT)
			;should	in the range "1024" to "65535" inclusive
			;for UDP based media

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

att-field ::=		1*(alpha-numeric)

att-value ::=		byte-string

sess-id	::=		1*(DIGIT)
			;should	be unique for this originating username/host

sess-version ::=	1*(DIGIT)
			;0 is a	new session

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

multicast-address ::=
			3*(decimal_uchar ".") decimal_uchar "/"	ttl
			[ "/" integer ]
			;multicast addresses may be in the range
			; to

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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

typed-time ::=		1*(DIGIT) [fixed-len-time-unit]

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

bwtype ::=		1*(alpha-numeric)

bandwidth ::=		1*(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 ">"

phone ::=		"+" POS-DIGIT 1*(space | "-" | DIGIT)
			;there must be a space or hyphen between the
			;international code and	the rest of the	number.

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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 ::=		byte-string
			;default is to interpret this as IS0-10646 UTF8
			;ISO 8859-1 requires a "a=charset:ISO-8859-1"
			;session-level attribute to be used

byte-string ::=		 1*(0x01..0x09|0x0b|0x0c|0x0e..0xff)
			;any byte except NUL, CR or LF

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


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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

email-safe ::=		safe | space | tab

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

space ::=		;ascii code 32
tab ::=			;ascii code 9
CRLF ::=		;ascii code 13 followed	by ascii code 10

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Appendix B: Guidelines for registering SDP names with IANA

There are five field names that	may be registered with IANA.  Using  the
terminology  in	 the  SDP  specification BNF, they are "media",	"proto",
"fmt", "att-field" and "bwtype".

"media"	(eg, audio, video, application,	data).

    The	set of media is	intended to be small  and  not	to  be	extended
    except  under  rare	 circumstances.	 The same rules	should apply for
    media names	as for top-level MIME content types, and where	possible
    the	 same  name should be registered for SDP as for	MIME.  For media
    other than existing	MIME top-level content types, a	 standards-track
    RFC	 MUST  be  produced  for  a  new  top-level  content  type to be
    registered,	and the	registration MUST provide good justification why
    no existing	media name is appropriate.


    In general this should be an IETF standards-track transport	protocol
    identifier such as RTP/AVP (rfc 1889 under the rfc 1890 profile).

    However, people will want to invent	their own proprietary  transport
    protocols.	 Some  of  these  should  be registered	as a "fmt" using
    "udp" as the protocol and some of which probably can't be.

    Where the protocol and the application are intimately  linked,  such
    as	with  the LBL whiteboard wb which used a proprietary and special
    purpose protocol over UDP, the protocol name should	be "udp" and the
    format  name  that should be registered is "wb".  The rules	for for-
    mats (see below) apply to such registrations.

    Where the proprietary transport protocol really  carries  many  dif-
    ferent  data formats, it is	possible to register a new protocol name
    with IANA.	In such	a case,	an RFC MUST be produced	 describing  the
    protocol  and  referenced  in  the registration.  Such an RFC MAY be
    informational, although it is preferable if	it is standards-track.


    The	format namespace is dependent on  the  context	of  the	 "proto"
    field,  so	a  format cannot be registered without specifying one or
    more transport protocols that it applies to.

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    Formats cover all the possible encodings that might	want to	be tran-
    sported in a multimedia session.

    For	RTP formats that have been assigned static  payload  types,  the
    payload  type  number is used.  For	RTP formats using a dynamic pay-
    load type number, the dynamic payload type number is  given	 as  the
    format and an additional "rtpmap" attribute	specifies the format and

    For	non-RTP	formats, any unregistered format name may be registered.
    If	there  is  a suitable mapping from a MIME subtype to the format,
    then the MIME subtype name should be registered.   If  there  is  no
    suitable  mapping  from  a	MIME  subtype,	a  new	name  should  be
    registered.	 In either case, unless	there are strong reasons not  to
    do	so, a standards-track RFC SHOULD be produced describing	the for-
    mat	and this RFC SHOULD be referenced in the registration.

"att-field" (Attribute names)

    Attribute field names MAY be registered with IANA, although	this  is
    not	compulsory, and	unknown	attributes are simply ignored.

    When an attribute is registered, it	must be	accompanied by	a  brief
    specification stating the following:

    o	  contact name,	email address and telephone number

    o	  attribute-name (as it	will appear in SDP)

    o	  long-form attribute name in English

    o	  type of attribute (session level, media level, or both)

    o	  a one	paragraph explanation of the purpose of	the attribute.

    o	  a specification  of  appropriate  attribute  values  for  this

    IANA will not sanity check such attribute  registrations  except  to
    ensure that	they do	not clash with existing	registrations.

    Although the above is the minimum that  IANA  will	accept,	 if  the
    attribute  is expected to see widespread use and interoperability is
    an issue, authors are encouraged to	produce	 a  standards-track  RFC
    that specifies the attribute more precisely.

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    Submitters of registrations	should ensure that the specification  is
    in	the spirit of SDP attributes, most notably that	the attribute is
    platform independent in the	sense that it makes no implicit	 assump-
    tions  about  operating systems and	does not name specific pieces of
    software in	a manner that might inhibit interoperability.

"bwtype" (bandwidth specifiers)

    A proliferation of bandwidth specifiers is strongly	discouraged.

    New	bandwidth specifiers may be registered with IANA.   The	 submis-
    sion  MUST	reference a standards-track RFC	specifying the semantics
    of the bandwidth specifier precisely, and indicating when it  should
    be used, and why the existing registered bandwidth specifiers do not

Registration Procedure

To register a name the above guidelines	should be followed regarding the
required  level	 of  documentation  that  is required.	The registration
itself should be sent to IANA.	Attribute registrations	 should	 include
the  information  given	 above.	  Other	registrations should include the
following additional information:

o     contact name, email address and telephone	number

o     name being registered (as	it will	appear in SDP)

o     long-form	name in	English

o     type of name ("media", "proto", "fmt" or "bwtype")

o     a	one paragraph explanation of the purpose of the	registered name.

o     a	reference to the specification (eg RFC number) of the registered

IANA may refer any registration	to the IESG or to any  appropriate  IETF
working	 group for review, and may request revisions to	be made	before a
registration will be made.

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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, Carsten Bormann and	Steve Hanna.


[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-

Handley/Jacobson				               [Page 39]
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octet Coded Character Set (UCS).

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

[9] F. Yergeau,	``UTF-8, a transformation  format  of  Unicode	and  ISO
10646'', RFC 2044, Oct 30th 1996

[10] ITU-T Recommendation H.332	(1998):	"Multimedia Terminal for Receiv-
ing Internet-based H.323 Conferences", ITU, Geneva.

[11] M.	Handley, E. Schooler, H. Schulzrinne, ``Session	Initiation  Pro-
tocol (SIP)'' Internet Draft, Nov 1997.

[12] H.	Schulzrinne, A.	Rao, R.	Lanphier, ``Real Time Streaming	Protocol
(RTSP)'' Internet Draft, Jan 1998.

Handley/Jacobson				               [Page 40]