Sessions Announcements

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Sessions Announcements

• Bernard Rapacchi
• Bernard.Rapacchi_at_urec.cnrs.fr
• FINE Workshop
• University College, London, 1st December 1997

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Finding Multimedia Sessions

• MBone is used extensively for multimedia
  conferencing
• Two basics ways to locate and to participate in a
  multimedia session
• Advertisement
• The session is advertised,
• Potential participants see the advertisement,
• Then they join the session to participate.
• Sessions are advertised by SAP, email, news
  group, web pages,
• Invitation
• Users are invited by others to participate in a
  session,
• The session may or may not be advertised.
• SIP is the invitation protocol.
• The MBone equivalent to the Phone Call

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Session Directory

• SDR ( MICE and MERCI Project, UCL)
• Advertised session (SAP)
• Invitation (quick call) (SIP)
• 4 default media types audio, video, whiteboard,
  text (SDP)
• Version number
• 2.3a1, november 1996
• 2.4a6, july 1997, incompatible with 2.2 et 2.3

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Background

• MBone site has to know
• conference s multicast group address
• UDP ports for data stream
• Medias information
• Time information
• 3 Protocols
• Session Description Protocol
• Session Announcement Procotocol
• Session Initiation Protocol

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SDP Session Description Protocol

• SDP is a session description protocol for
  multimedia sessions
• Common usage is for a client to announce a
  conference session
• Periodically multicast an announcement packet to
  a well known address and port (SAP)
• Email and WWW Announcements
• MIME content type application/sdp
• but the receiver of an announcement cannot
  necessarly receive the session

SAP Header
Text payload ..
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SDP requirement

• SDP serves two primary purposes
• a means to communicate the existence of a session
• a means to convey information to enable joining
  and participating
• SDP includes
• Session name and purpose
• Period(s) of time the session is active
• The media comprising the session
• Information to receive those media (adresses,
  ports, formats,)
• plus
• Information about the bandwith to be used
• Contact information for the person responsible
  for the session

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SDP Timing Information

• Sessions may be bounded or unbounded in time
• SDP can convey
• An arbitrary list of start and stop times
  bounding the session
• For each bound, repeat times such as every
  Wednesday at 10am for one hour 
• Timing information is globally consistent,
  irrespective of local time zone

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SDP more

• Private sessions 
• encrypting the session description
• use of private announcement to convey encryption
  keys
• Further information about a session
• SDP may inlcude additionnal pointer in the form
  of URI
• Categorization
• to filter announcements that are of interest
• Internationalization
• SDP recommends the use of the ISO 10646 character
  set
• SDP also allows other character sets such as ISO
  8859-1

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SAP Session Announcement Protocol

• SAP client announces a conference session by
• periodicaly multicasting an announcement packet
  to a well known address and port number
• 224.2.127.254/9875 (sap.mcast.net)
• With the same scope (TTL) as the session
• Re-advertisement is dependent on 3 factors
• the scope (TTL) of the session
• number of other sessions currently being
  announced
• the size of your data packets

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SAP Session Deletion

• Explicit Timeout
• if current time is later than the end-time for
  the session
• if a session deletion packet is received
• specifying the version of the session to be
  deleted
• same IP-source address
• Implicit Timeout
• Session announcement message should be received
  periodicaly
• The announcement period can be predicted by the
  receiver
• If a session announcement message has not been
  received for ten times the period or half an hour
  then the session is deleted

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SAP Session Modification

• A pre-announced session can be modified by simply
  announcing the modified session description
• The session itself is uniquely identified by the
  SDP origin field in the payload
• Same rules apply for session modification as for
  session deletions
• If cached session and modificated session do not
  have same authentification or IP-source addresse,
  the new session has to be treated as a new
  announcement and displayed in addition

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SIP Session Initiation Protocol

• SIP does not prescribe how a conference is to be
  managed
• SIP can invite users to conferences by conveying
  the information necessary.
• SIP does not allocate multicast addresses, this
  is done by SAP.
• SIP uses a central server to manage conference
  and participant state and distribute state via
  multicast.

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SIP Addressing

• Host-specific addresses user_at_host
• user is an operating system user name
• host is either a domain name having a DNS A
  record or a numeric network address
• e.g.
• br_at_sylvestre.grenoble.urec.cnrs.fr
• tuy_at_195.220.197.12
• Host independent addresses user_at_location
• user may be any unambiguous name including
  aliases
• location domain name having an MX or A record
• e.g.
• Bernard.Rapacchi_at_urec.cnrs.fr
• SIP uses proxy servers to locate the user

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Bibliography

• Internet Drafts
• draft-ietf-mmusic-sap-00.txt 19th Nov 1996
• draft-ietf-mmusic-sap-sec-01.txt 29th Jul 1997
• draft-ietf-mmusic-sdp-04.txt 2nd Sep 1997
• draft-ietf-mmusic-sip-04.txt 13rd Nov 1997
• draft-ietf-mmusic-confarch-00.txt 18th Sep 1997
• WWW sites
• http//www-mice.cs.ucl.ac.uk/merci/
• http//mirage.informatik.uni-mannheim.de/mmbook/bo
  ok.html(Internetworking Multimedia, UCL Press)