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

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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.

Handley/Jacobson                                               [Page 29]

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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)

Handley/Jacobson                                               [Page 30]

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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"

Handley/Jacobson                                               [Page 31]

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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

Handley/Jacobson                                               [Page 32]

INTERNET-DRAFT                                             22nd Jan 1997

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.

Handley/Jacobson                                               [Page 33]

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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
                        | POS-DIGIT 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

DIGIT ::=               0 | POS-DIGIT

Handley/Jacobson                                               [Page 34]

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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

Handley/Jacobson                                               [Page 35]

INTERNET-DRAFT                                             22nd Jan 1997

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.

Handley/Jacobson                                               [Page 36]

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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.

Handley/Jacobson                                               [Page 37]

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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.

Handley/Jacobson                                               [Page 38]

INTERNET-DRAFT                                             22nd Jan 1997

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]

INTERNET-DRAFT                                             22nd Jan 1997

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]