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INTERNET DRAFT                                            FUJIKAWA Kenji
draft-fujikawa-sdp-url-01.txt                             KURIYA Shinobu
                                                        Kyoto University
                                                           7 August 1998

                             SDP URL Scheme



Status of this Memo

   This document is an Internet-Draft.  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
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   To view the entire list of current Internet-Drafts, please check the
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   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).



Abstract

   This document describes a format for an Session Description Protocol
   Uniform Resource Locator (SDP URL) which will allow Internet clients
   to have direct access to multimedia sessions.



1. Introduction

   SDP[1] is a format for describing information of multimedia sessions,
   and generally is conveyed by SAP (Session Announcement Protocol)[4].
   However, SDP is just a format for session description and is intended
   to use different transport protocols including SAP, E-mail and World
   Wide Web (WWW).

   An SDP file has to be saved in a WWW server for distributing SDP
   messages when utilizing WWW.  At first, Internet clients have access
   to a WWW server to get an HTML file that has the URL of the SDP file.



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   Clients have to have another access to the server to get the SDP file
   to start a multimedia session.

   SDP URL scheme eliminates the second unnecessary access to a WWW
   server.  Since SDP URL is written directly on an HTML file, a session
   can be started only by access to the HTML file with SDP URL.  It is
   not necessary to get another access to the server.

   When a user clicks SDP URL in an HTML file, a WWW browser
   automatically launch applications for participation in the session.
   This is equal to selecting a session on an application such as sdr.



2. Description of the SDP Scheme

   The SDP URL scheme is basically the same format as that of the
   original SDP except for a few points.

   A whitespace is replaced by '+', a return by '&', and the characters
   reserved in RFC 1738[2] and a original '+' by ascii code described by
   %xx.  The term "RTP/AVP" which specifies a transport protocol is
   replaced by the "RTP-AVP".  The <type> of  'v', 'o' and 's' which
   cannot be omitted in the original SDP can be omitted.  If 'v' is
   omitted, then the SDP version of an SDP URL is regarded as 1.

   URL is generally written in the format:

       <scheme>://<address>:<port>/<url-path>

   SDP's connection information and session name correspond to <address>
   and <url-path> respectively.

   An SDP URL takes the form:

       sdp://[ <address> [:ttl=<ttl>] [:noa=<noa>] ] / [<sessionname>]
             [ #<type>=<value> *[&<type>=<value>] ]

   The <address> is the connection address that will be either a unicast
   IP address or a class-D IP multicast group address.  The <ttl> (time-
   to-live) defines the scope with which multicast packets sent in a
   session should be sent when the <address> is a multicast address.
   The <ttl> is ignored when the <address> is a unicast address.  The
   value of <ttl> takes 1 when the <ttl> is omitted.  The <noa> (number-
   of-addresses) is the number of multicast addresses contiguously
   allocated above the base address <address>.

   These can also be described as a connection information 'c' and the



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   <sessionname> as a session name 's'.  The <type>, the <value> and
   their order follow [1].



3. Examples

   The followings are some example SDP URLs using the format defined
   above. A multimedia session is on a multicast address 224.192.2.3,
   the session name "sdp test", TTL 16 and port 10000 .  The media type
   is audio, profile AVP and payload type PCM.

       sdp://224.192.2.3:ttl=16/sdp+test
       #m=audio+10000+RTP-AVP+0

   This could also be written in

       sdp:///#s=sdp+test&c=IN+IP4+224.192.2.3%2f16&m=audio+10000+RTP-AVP+0

   The former description is preferable for look, suiting to the URL
   convention.

   Multicast addresses are registered in DNS in [3].  In this case, the
   URL is also written as:

       sdp://london-station.bbcc.com:ttl=16/sdp+test
       #m=audio+10000+RTP-AVP+0

   where ``london-station.bbcc.com'' is the domain name of the address
   224.192.2.3.

   Some services may use additional data.  The time that a session is
   active is specified.  Both audio and video are used.  All media
   applications are requested to be receive-only and the maximum
   framerate of video to be 10.

       sdp://london-station.bbcc.com:ttl=16/sdp+test
       #t=2873397496+2873404696&a=recvonly
       &m=audio+10000+RTP-AVP+0
       &m=video+9999+RTP-AVP+31&a=framerate:10



5. Considerations to Scope Rule

   It is asserted that announcements of multicast sessions made via WWW
   cause the mismatch between the scope where WWW is valid and the scope
   restricted by the multicast addresses. [1] However, Ohta shows that



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   this is not a problem but the idea of scope has an essential problem.
   [3]


6. QoS Specifications

   The notation of RSVP parameters should be defined in [1] or in this
   draft for the purpose of specifying Quality of Services (QoS).



7. Proposed Syntax

   The proposed BNF syntax is encoded as specified in RFC 1738 [2].

       sdpurl          = "sdp://" [ connection ] "/" [ sessionname ]
                         ["#" parameter *["&" parameter ] ]

       connection      = address [ ":ttl=" ttl ][ ":noa=" noa ]
       address         = addressname | addressnumber
       addressname     = *[ domainlabel "." ] toplabel
       domainlabel     = alphadigit |
                         alphadigit *[ alphadigit | "-" ] alphadigit
       toplabel        = alpha | alpha *[ alphadigit | "-" ] alphadigit
       alphadigit      = alpha | digit
       addressnumber   = digits "." digits "." digits "." digits
       ttl             = digits
       noa             = digits
       digits          = 1*digit

       sessionname     = 1*uchar

       parameter       = type "=" value
       type            = alpha
       value           = 1*uchar [ ":" attributevalue]
       attributevalue  = 1*uchar

       alpha, digit and uchar are defined in RFC 1738.



References

[1]    M. Handley and V. Jacobson, "SDP: Session Description Protocol",
       RFC 2327, Nov 1997.

[2]    T. Berners-Lee, L. Masinter and M. Mccahill, "Uniform Resource
       Locators (URL)", RFC 1738, Dec 1994.



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[3]    M. Ohta, J. Crowcroft, "Static Multicast", Internet Draft draft-
       ohta-static-multicast-00.txt (work in progress), March 1998.

[4]    M. Handley, "SAP: Session Announcement Protocol", Internet Draft
       draft-ietf-mmusic-sap-00.txt (work in progress), Nov 1996.




Security Considerations

   (to be written)


Appendix


A. Implementation

   Implementation of an SDP URL interpreter for Emacs/WWW is available
   at
   "http://www.lab1.kuis.kyoto-u.ac.jp/members/magician/sdp-url-0.1.tar.gz".


Authors' Address

   FUJIKAWA Kenji
   Graduate School of Informatics,
   Kyoto University
   Yoshidahonmachi, Sakyo-Ku, Kyoto City, 606-01, Japan
   Phone : +81 75-753-5387
   Email : magician@kuis.kyoto-u.ac.jp

   KURIYA Shinobu
   Department of Information Science,
   Faculty of Engineering, Kyoto University
   Yoshidahonmachi, Sakyo-Ku, Kyoto City, 606-01, Japan
   Phone : +81 75-753-5387
   Email : kuriya@kuis.kyoto-u.ac.jp












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