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 time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as ``work in progress.'' To view the entire list of current Internet-Drafts, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific 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. FUJIKAWA Kenji Expires on 6 February 1999 [Page 1]
INTERNET DRAFT SDP URL scheme August 1998 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 FUJIKAWA Kenji Expires on 6 February 1999 [Page 2]
INTERNET DRAFT SDP URL scheme August 1998 <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 FUJIKAWA Kenji Expires on 6 February 1999 [Page 3]
INTERNET DRAFT SDP URL scheme August 1998 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. FUJIKAWA Kenji Expires on 6 February 1999 [Page 4]
INTERNET DRAFT SDP URL scheme August 1998 [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 FUJIKAWA Kenji Expires on 6 February 1999 [Page 5]