Internet Engineering Task Force                                MMUSIC WG
INTERNET-DRAFT                                        Mark Handley/ACIRI
draft-ietf-mmusic-sdp-new-00.txt              Van Jacobson/Packet Design
                                                       Colin Perkins/ISI
                                                        17 November 2000
                                                       Expires: May 2001

                   SDP: Session Description Protocol

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at


     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

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list at and/or the authors.

1.  Introduction

Note: This draft is essentially identical to RFC 2327. It is made
available to stimulate discussion of corrections and clarifications
which need to be made in order to advance SDP to a draft standard RFC.

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
participation.  This document defines a session description protocol for
this purpose, and for general real-time multimedia session description
purposes.  This draft does not describe multicast address allocation or
the distribution of SDP messages in detail.  These are described in
accompanying drafts.  SDP is not intended for negotiation of media

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
extensively for multimedia conferencing.  Such conferences usually have
the property that tight coordination of conference membership is not
necessary; 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
protocols 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 Protocol.

SDP is intended to be general purpose so that it can be used for a wider
range of network environments and applications than just multicast
session 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.

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3.  Glossary of Terms

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

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

    A multimedia session is a set of multimedia senders and receivers
    and the data streams flowing from senders to receivers.  A
    multimedia 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
    discover and participate in a multimedia session.

3.1.  Terminology

This document uses the same words as RFC 1123 for defining the
significance of each particular requirement.  These words are:

     This word or the adjective ``required'' means that the item is an
     absolute requirement of the specification.

     This word or the adjective ``recommended'' means that there may
     exist valid reasons in particular circumstances to ignore this
     item, but the full implications should be understood and the case
     carefully weighed before choosing a different course.

     This word or the adjective ``optional'' means that this item is
     truly optional.  One implementation may choose to include the item
     because a particular application requires it or because it enhances

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     the product, for example, another implementation may omit the same

An implementation is not compliant if it fails to satisfy one or more of
the must requirements for the protocols it implements.  An
implementation that satisfies all the must and all the should
requirements for the protocols it implements is said to be
``unconditionally compliant''; one that satisfies all the must
requirements but not all of the should requirements for the protocols it
implements is said to be ``conditionally compliant''.

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
multicast address and port using the Session Announcement Protocol

SAP packets are UDP packets with the following format:

      0                   31
      | SAP header         |
      | text payload       |

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

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

4.2.  Email and WWW Announcements

Alternative means of conveying session descriptions include electronic
mail and the World Wide Web.  For both email and WWW distribution, the
use of the MIME content type ``application/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.

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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
multimedia 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
communicate 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

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

o   Information about the bandwidth to be used by the conference

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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
performed 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
information 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
categorisation mechanism for sessions that is capable of being

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
character 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
announcements 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
character 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
description starts with an `m=' line and continues to the next media
description 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
descriptions 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
session directory must ignore any attribute it doesn't understand.

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

An example SDP description is:

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     o=mhandley 2890844526 2890842807 IN IP4
     s=SDP Seminar
     i=A Seminar on the session description protocol
     u= (Mark Handley)
     c=IN IP4
     t=2873397496 2873404696
     m=audio 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
encoding, 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
detect which of several announcements for the same session is the most

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recent.  Again its usage is up to the creating tool, so long as
<version> is increased when a modification is made to the session data.
Again, it is recommended (but not mandatory) that an NTP timestamp is
used.  <network type> is a text string giving the type of network.
Initially ``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 either the fully-qualified domain name
of the machine, or the dotted-decimal representation of the IP version 4
address of the machine.  For an address type of IP6, this is either the
fully-qualified domain name of the machine, or the compressed textual
representation of the IP version 6 address of the machine.  For both IP4
and IP6, the fully-qualified domain name is the form that SHOULD be
given unless this is unavailable, in which case the globally unique
address may be substituted.  A local IP address MUST NOT be used in any
context where the SDP description might leave the scope in which the
address is meaningful.

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

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for feedback and questions.



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

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

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

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

Email Address and Phone Number

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

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

o   Either an email field or a phone field must be specified.
    Additional 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

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    person who may be contacted.  This should be enclosed in parenthesis
    if it is present.  For example: (Mark Handley)

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

        e=Mark Handley <>

    The free text string should be in the ISO-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
    connection address contains the unicast IP address of the expected
    data source or data relay or data sink as determined by additional
    attribute 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.

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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.

    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
    including 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.


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

o   <bandwidth-value>  is in kilobits per second

o   <modifier>  is 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
      administrative scope region (the Mbone bandwidth vs. TTL limits
      are given in the MBone FAQ).  If the bandwidth of a session or
      media in a session is different from the bandwidth implicit from
      the scope, a `b=CT:...' line should be supplied for the session
      giving the proposed 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

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    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
    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

    The general assumption may be made, when displaying unbounded
    sessions 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

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    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.
    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,
    different 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
    session 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

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

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

Encryption Keys

k=<method>:<encryption key>

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
        prohibited 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
        containing 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

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        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.



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

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

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

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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
inadvertent collision with registered attributes.  In either case, if an
attribute is received that is not understood, it should simply be
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
    multicasting 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
    transport ports.  This is done using a similar notation to that used

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    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
    49172 and 49173 form the second RTP/RTCP pair.  RTP/AVP is the
    transport 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
    protocol values are dependent on the address-type field in the
    ``c='' fields.  Thus a ``c='' field of IP4 defines that the
    transport protocol runs over IP4.  For IP4, it is normally expected
    that most media traffic will be carried as RTP over UDP.  The
    following transport protocols are preliminarily defined, but may be
    extended through registration of new protocols with IANA:

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

    - udp  - User Datagram Protocol

    If an application uses a single combined proprietary media format
    and transport protocol over UDP, then simply specifying the
    transport protocol as udp and using the format field to distinguish
    the combined protocol is recommended.  If a transport protocol is
    used over UDP to carry several distinct media types that need to be
    distinguished by a session directory, then specifying the transport
    protocol and media format separately is necessary.  RTP is an
    example 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-

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    protocol-specific but format-independent are possible.

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

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

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

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

    For media whose transport protocol is RTP, SDP can be used to
    provide 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
    payload types.

    An example of a static payload type is u-law PCM coded single
    channel 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:

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         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
    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
    session 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.

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    Application Formats:

    wb:   LBL Whiteboard (transport: udp)

    nt:   UCL Network Text Editor (transport: udp)

Suggested Attributes

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

    This attribute gives the dot-separated hierarchical category of the
    session.  This is to enable a receiver to filter unwanted sessions
    by category.  It would probably have been a compulsory separate
    field, except for its experimental nature at this time.  It is a
    session-level attribute, and is not dependent on charset.

    Like the cat attribute, this is to assist identifying wanted
    sessions 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 specified 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

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    mode.  This is necessary for interactive conferences with tools such
    as wb which defaults to receive only mode. 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-only mode.
    An example may be where a different unicast address is to be used
    for a traffic destination than for a traffic source. In such a case,
    two media descriptions may be use, one sendonly and one recvonly. It
    can be either a session or media attribute, but would normally only
    be used as a media attribute, 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
    coupled 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
    dependent 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:


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    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
    character 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.

    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
    session 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 in 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

    In general, sending session descriptions consisting of multiple
    languages should be discouraged.  Instead, multiple descriptions
    should be sent describing the session, one in each language.
    However this is not possible with all transport mechanisms, and so
    multiple sdplang attributes are allowed although not recommended.

    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
    sufficient 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
    specifies 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

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    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
    cannot 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
    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
    suggested 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.

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6.1.  Communicating Conference Control Policy

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

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


In some cases, however, it is possible that this may be insufficient to
communicate the details of an unusual conference control policy.  If
this is the case, then a conference attribute specifying external
control 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
controller 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
implications 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
different transport protocols may be used to distribute session
description, and the nature of the authentication will differ from
transport to transport.

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
INAPPROPRIATE 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
arriving by session announcement, email, sessioR multimedia,session page
SHOULD not deliver the user into an interactive
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
circumstances 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
INAPPROPRIATE for a firewall to open such holes for unicast data streams

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unless the session description comes in a request from inside the
firewall.  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 global multicast sessions from local ones.

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Appendix A: SDP Grammar

This appendix provides an Augmented BNF grammar for SDP.  ABNF is
defined in RFC 2234.

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 1*(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 =                FQDN | unicast-address

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