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Sessions Announcements

Sessions Announcements. Bernard Rapacchi Bernard.Rapacchi@urec.cnrs.fr FINE Workshop University College, London, 1st December 1997. Finding Multimedia Sessions. MBone is used extensively for multimedia conferencing Two basics ways to locate and to participate in a multimedia session :

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Sessions Announcements

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  1. Sessions Announcements Bernard Rapacchi Bernard.Rapacchi@urec.cnrs.fr FINE Workshop University College, London, 1st December 1997

  2. 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 2

  3. 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 3

  4. 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 10

  5. 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 …………….. 11

  6. 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 12

  7. 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 13

  8. 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 14

  9. 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 15

  10. 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 16

  11. 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 17

  12. 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. 18

  13. SIP : Addressing • Host-specific addresses : user@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@sylvestre.grenoble.urec.cnrs.fr • tuy@195.220.197.12 • Host independent addresses : user@location • user : may be any unambiguous name including aliases • location : domain name having an MX or A record • e.g. : • Bernard.Rapacchi@urec.cnrs.fr • SIP uses proxy servers to locate the user 19

  14. 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/book.html(Internetworking Multimedia, UCL Press) 20

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