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RFC 2122
Network Working Group D. Mavrakis
Request for Comments: 2122 Monaco Telematique MC-TEL
Category: Standards Track H. Layec
ETSI
K. Kartmann
Telecommunication+Multimedia
March 1997
VEMMI URL Specification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
1) Abstract
A new URL scheme, "vemmi" is defined. It allows VEMMI client software
and VEMMI terminals to connect to multimedia interactive services
compliant to the VEMMI standard (Enhanced Man-Machine Interface for
Videotex and Multimedia/Hypermedia Information Retrieval Services),
sometimes abbreviated as "VErsatile MultiMedia Interface".
VEMMI is a new international standard for on-line multimedia
services, that is both an ITU-T (International Telecommunications
Union, ex. CCITT) International Standard (T.107) [2] and an
European Standard (ETSI European Telecommunications Standard
Institute) standard (ETS 300 382 [3], obsoleted by ETS 300 709 [1]).
VEMMI could be described as an on-line multimedia protocol describing
both the man-machine interface and the client/server exchange
protocol. VEMMI operates usually on a single continuous session
between a client and a host and supports an object-oriented, event-
driven, client/server oriented and platform independent multimedia
interface. The well-known tcp port number 575 has been assigned by
IANA to the VEMMI protocol [13].
A description of the VEMMI standard along with its last approved
version is publicly available on the Web: see the URL
http://www.etsi.fr/ecs/projects/vemmi/vemmi.htm). A presentation of
VEMMI can be found on http://www.mctel.fr/VEMMI/vemmien_intro.html
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RFC 2122 VEMMI URL Specification March 1997
2) VEMMI URL scheme utility and operability:
- VEMMI service selection: A VEMMI multimedia server will listen on
its VEMMI well-known port for incoming connections. The server
could of course be engaged in many simultaneous connections, and
after a connection is established, the terminal must be able to
select the desired VEMMI application, as a same system may host
different VEMMI applications.
The best mechanism to fully describe the VEMMI service to activate
is the URL mechanism.
- Reporting user action to a remote server: The VEMMI protocol
establishes a continuous TCP/IP link between the terminal and the
server during the whole user session. During a session managing
VEMMI objects, the user actions are usually reported back to the
server using the VEMMI user data report mechanism that is an
integral part of the VEMMI protocol.
However, in some case, the URL mechanism may be required to send
back reports to a remote host. VEMMI is for example able to display
HTML documents within a multimedia display area in a VEMMI dialog
box. these HTML documents may be managed either by the VEMMI server
(acting as a proxy gateway) or directly by the client software that
will issue itself the HTTP requests on the network and browse
across documents on the Web as a standard Web browser (the link to
the VEMMI server is kept and used for interacting with other VEMMI
objects and components but the VEMMI server may not be informed of
the user navigation on the Web inside the multimedia area).
In such a case, the URL mechanism could also be used to report the
user actions and commands within a HTML document to be reported to
the VEMMI server or even another system.
- Extension of Web browsers: The VEMMI protocol is quite
complementary to HTTP/HTML used by Web browsers, and several
networks operators have decided to support jointly Web and VEMMI
(seen as complementary protocols). Thanks to the VEMMI URL, Web
browsers will be able to activate a VEMMI client software module to
start a VEMMI session to the requested service when the user
activate a vemmi URL included in the HTML document.
3) Description of the VEMMI scheme
The VEMMI URL scheme is used to designate multimedia interactive
services conforming to the VEMMI standard (ITU/T T.107 and ETS 300
709).
A VEMMI URL takes the form:
vemmi://<host>:<port>/<vemmiservice>;
<attribute>=<value>
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RFC 2122 VEMMI URL Specification March 1997
as specified in Section 3.1. of RFC 1738. If :<port> is omitted, the
port defaults to 575 (client software may choose to ignore the
optional port number in order to increase security). The
<vemmiservice> part is optional and may be omitted.
This URL does not designate a data object, but rather a multimedia
interactive service. A VEMMI service starts a multimedia session
managing multimedia objects and interacting with the user during the
session. To the difference of other stateless protocols, the link
between the client and the server is usually maintained during the
whole session (although in some cases other mechanisms may be used,
see below).
The <vemmiservice> is the name of the VEMMI service to activate. This
field is not mandatory and could be omitted for example if the remote
host manages only one VEMMI service or activates a VEMMI service by
default when no service is specified. If this field is omitted in the
URL and the server requests it, it is assumed that the VEMMI client
software will prompt the user for it.
In addition, after the <vemmiservice>, optional attributes and values
(parameters) associated with the VEMMI service may be specified as
part of the URL. When present, each parameter (attribute/value pair)
is separated from each other and from the rest of the URL by a ";"
(semicolon). The name of the attribute and its value are separated by
a "=" (equal sign). If present, these fields are used to transmit
additional data useful for service selection or to request to perform
a specific processing. For example, the $USERDATA field can be
specified to transmit additional user-specified data to the VEMMI
service.
4) Client/server dialog during service selection
The VEMMI client will interpret the VEMMI URL to connect to the
remote host and to access the specified VEMMI service. After
connecting to the remote system, the host may prompt the VEMMI client
for service name and user identification.
Before starting the VEMMI session, a VEMMI terminal is in 'standard'
mode and may display the data received from the network in a videotex
or telnet terminal window. As the VEMMI session may be started
anytime during an interactive videotex or telnet session, the VEMMI
service selection is performed by a simple dialog between the client
and the server.
The service, username and password information are transmitted by the
client software to the host in answer to the corresponding requests
received from the host. The following behavior is expected from the
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RFC 2122 VEMMI URL Specification March 1997
client:
- wait for the optional request strings from the host server
('service:', 'username:' and 'password:') and answer them
(respectively by <vemmiservice> value defined in the URL and the
<username> and <password> entered by the user if required). The
terminal answer may be sent automatically if the answers are known
(that is if they are specified in the URL or terminal
configuration) or it may prompt the user for the needed
informations. When parameters (attribute and value pairs) are
present in the URL, these fields will be sent following the
<vemmiservice> transmitted to the host in answer to the 'service:'
request received from the host, separated from the <vemmiservice>
value by a semicolon.
- the client answers must always be followed by a Carriage Return
(CR). If a Line Feed (LF) is transmitted after the CR, it will be
ignored by the server.
- in both cases, the server may echo the characters received from the
client terminal, the received CR being echoed as CR LF. The server
may echo the password characters as stars or any other scrambled
output for safety purpose.
- the client must also be ready to start the VEMMI session as soon
as it receives the VEMMI_Open command. Before starting this VEMMI
session, the terminal is in 'standard' mode and may display the
data received from the network in a videotex or telnet terminal
window (this is the reason why the service, username and password
data are requested by the server using a telnet or videotex
compatible dialog).
Should an error occur during VEMMI service activation, the remote
host may inform the user by displaying the error cause. It is
recommended that, when possible and applicable, the status code
syntax described in HTTP [8] [9] be used to facilitate automatic
processing by the client of the host answer during error or normal
operation.
If a VEMMI client software wants to request a VEMMI object without
establishing a continuous VEMMI session, such a request may also be
performed using a HTTP request for the vemmi object encoded using the
Internet media type application/vemmi (pending registration by IANA
[10]). In the same way, HTTP could be used in some cases to exchange
data pertaining to a VEMMI session with or without setting the keep-
alive keyword in the Connection header to request a persistent
connection [9]. Protocol switching using the upgrade header field may
be used in such case to switch to vemmi protocol [9]. This possible
use of HTTP for VEMMI is not described in this document.
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RFC 2122 VEMMI URL Specification March 1997
5) Proposed syntax
The proposed BNF syntax is encoded as specified in RFC 1738 [5] [14]:
; vemmi (see ITU-T T.107 and ETSI ETS 300 709)
vemmiurl = "vemmi://" hostport [ "/" vemmiservice *[ parameter ] ]
vemmiservice = *[ uchar | "/" | "?" | ":" | "@" | "&" | "=" ]
parameter = ";" attribute "=" value
attribute = *[ uchar | "/" | "?" | ":" | "@" | "&" ]
value = *[ uchar | "/" | "?" | ":" | "@" | "&" ]
This syntax: - allows the user to specify the remote host address by
its name or numeric address. Although he could select a specific
port, it is expected the information providers and VEMMI software
will mostly use the port number assigned to VEMMI (575) [13]. For
security reasons, the username and password could not be specified
in the URL.
- allows him to select a specific VEMMI service if the remote host
manages several different VEMMI services.
- allows also to send additional data to the service using the
parameter syntax, either during the service selection phase or when
the user selects a vemmi hyperlink within a HTML document displayed
in a VEMMI multimedia area. To the difference of the params syntax
used in [4], the parameter syntax requires each value to be labeled
by an attribute. The parameter attribute names are case-
insensitive. Parameter values may or may not be case-sensitive,
depending on the attribute.
All the values of fieldname beginning by a dollar ($) sign are
reserved for specific use, including:
- $COMMAND: VEMMI command to be returned to the host when the VEMMI
session do not use a continuous link.
- $CONTEXTDATA: context data.
- $OBJECT_REQUEST: requests the retransmission of a given VEMMI object.
- $USERDATA: user data specific by the user and to be processed by the
VEMMI service.
6) Examples:
Some examples of VEMMI URLs along with the corresponding
client/server dialog are presented below, they are for information
only:
a) A simple VEMMI URL and the corresponding dialog for a VEMMI
service that does not enforce access control might be:
URL: vemmi://zeus.mctel.fr/demo
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RFC 2122 VEMMI URL Specification March 1997
In this case, the exchange between client and server will be as
follow (the server requests are presented left, client answers
right):
service: demo
200 OK {status code returned by the VEMMI host}
b) The service name may be omitted (for example if the remote server
hosts only one VEMMI service), and the URL might then be:
URL: vemmi://zeus.mctel.fr
In this case, the VEMMI interactive session is started immediately
by the host without requesting first the service name (should the
client receive a service request from the host, it will prompt the
user for service name).
c) The URL could not include the username and password. In this case,
should they be requested by the host, the VEMMI client may use a
default value specified for this service or prompt the user for
them (for example it could propose anonymous and user e-mail
address as defaults):
URL: vemmi://mctel.fr/demo
In this case, the exchange between client and server may be as
follows (server requests at the left, client answers at the right):
service: demo
login: anonymous {user has been prompted for username}
password: mavrakis@ties.itu.ch {user prompted for password}
401 Unauthorized {an anonymous user is not allowed to
access the service}
d) Some services may use additional data transmitted in the parameter
fields, for example:
URL: vemmi://mctel.fr/demo;$USERDATA=smith;account=1234
If no access check is done by the host, the dialog might be:
service: demo;$USERDATA=smith;account=1234
200 OK
...starting VEMMI session...
7) Procedure to use when a VEMMI URL is encountered in a HTML document
without VEMMI support:
The VEMMI URL support may be built-in in some Web browsers, or
offered by an associated software or plug-in interworking with the
user browser, for example using the WWW_RegisterProtocol API command
to register the new VEMMI protocol.
When a Web browser encounters a VEMMI URL without having VEMMI support,
two cases may occur:
- some browsers will detect an unrecognized scheme and signal an
unrecoverable error directly.
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RFC 2122 VEMMI URL Specification March 1997
- others will manage it as a relative URL [4] and will build a
complete URL including the VEMMI URL and will request it from the
host having sent the current document. In this case the host will
usually return the error "not found".
Among the mechanisms that could be used in order to offer a friendly
interface to both users with and without VEMMI support:
- when the second case occurs and the relative URL including the
vemmi:// string is transmitted to the server, the HTTPD server may
be modified in order to recognize such URL and to propose the
downloading of a VEMMI client software.
- the HTML document including the vemmi URL allowing to start the
VEMMI session may propose both options, for example:
If your browser supports VEMMI, directly
<A HREF="vemmi://ares.mctel.fr/TEST">start the interactive
multimedia service</A>, otherwise
<A HREF="ftp://ftp.mctel.fr/vemmi.exe">download first a VEMMI
client software</A>.
- the application/vemmi MIME type is defined below (to allow for
example exchange of vemmi objects). A possible way is for the
server to look in the HTTP Accept header field and to deduce that if
application/vemmi is supported, then the VEMMI support exists (in
this case, application/vemmi is to be defined in the browser and
associated with the vemmi decoder).
8) Security Considerations:
The VEMMI URL scheme is subject to the same security implications as
the general URL scheme [5] [14], so the usual precautions outlined in
[5] [14] apply (for example, it is not allowed to store the username
and password in the URLs).
Furthermore, among VEMMI objects that could be used during the
interactive session, metacode objects (representing a sequence of
VEMMI commands) and operative objects (they are executable programs
to be run on the client platform) may be downloaded and/or started.
In order to protect the user against the activation of an harmful
operative object, it is strongly recommended that the users use the
configuration menu of their VEMMI software to disable the option of
running operative objects when receiving potentially unsafe VEMMI
objects, or at least enable the option to request first user approval
before starting the execution of an operative object.
The VEMMI remote interactive services may vary widely in their access
control policies; in practice, when a prompt for username or password
is received, the VEMMI terminal should request them from the user.
The VEMMI terminal implementation could support additional features,
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RFC 2122 VEMMI URL Specification March 1997
for example proposing by default "anonymous" as username and the user
Internet e-mail address as password, or looking in an encrypted local
database for user identification on this service.
Such an identification mechanism using the username/password scheme
is unsecure and is provided for backwards compatibility only. The
VEMMI services requiring a safe identification procedure must rely on
other alternative mechanisms (e.g. S/KEY or other). In numerous
cases, the user identification procedure will be performed by the
VEMMI service.
9) application/vemmi MIME type
VEMMI is a multimedia interactive service and VEMMI objects are
usually exchanged through a continuous VEMMI multimedia session.
However, VEMMI objects could also be transmitted and exchanged using
other mechanisms, for example using HTTP, through e-mail, and so
on... The assignment of a MIME media type application/vemmi will
allow this transport and exchange of VEMMI objects, and this
paragraph describes this MIME type.
Furthermore, for Web browsers not supporting the addition of new URL
protocol scheme, the VEMMI MIME type may also be used to transmit,
instead of a VEMMI object, a text file containing the VEMMI URL to
activate to connect to a VEMMI server.
9A) DESCRIPTION:
MIME media type name: "application"
MIME subtype name: "vemmi"
Required parameters: none
Optional parameters:
- version:
an optional version number may be specified, in the format:
version=<integer>
The version number is a numeric integer whose is encoded as the
<version> parameter defined in ETS 300 709 (e.g. version=100), and
whose the first digit represents the major VEMMI version number.
It must be pointed out that the VEMMI objects includes the VEMMI
version and a timestamp.
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RFC 2122 VEMMI URL Specification March 1997
9B) ENCODING CONSIDERATIONS:
The "base64" mechanism is preferred because VEMMI use a native 8-bit
binary file format. However, as VEMMI includes its own 7-bits
encoding mechanisms, VEMMI data could also be transmitted in 7-bit
mode.
9C) SECURITY CONSIDERATIONS:
Refer to paragraph 8.
9D) INTEROPERABILITY CONSIDERATIONS:
VEMMI is designed to be fully platform independent, and the VEMMI
objects and contents could interoperate between any platform. The
only exception are the VEMMI operative objects that could be binary
programs specific to a given hardware platform and operating system.
10) Liaison address:
For all technical questions regarding this request, please contact:
Daniel Mavrakis
Monaco Telematique MC-TEL
P.O. Box 225
MC 98004 Monte-Carlo Cedex
PRINCIPALITY OF MONACO
EMail: Mavrakis@mctel.fr
Tel: (+377) 9216 8860
Fax: (+377) 9330 4545
Comments may also be addressed to:
Mr. Herve Layec,
ETSI STC TE1
06921 SOPHIA ANTIPOLIS Cedex
FRANCE
EMail: herve.layec@dri.france-telecom.fr
Tel: (+33) 2 99 12 73 01
Fax: (+33) 2 99 38 49 61
Mavrakis, et. al. Standards Track [Page 9]
RFC 2122 VEMMI URL Specification March 1997
Mr. Kurt Kartmann
Consulting
Telecommunication+Multimedia
Gabelsbergerstr. 2
D-64807 DIEBURG
GERMANY
EMail: k.kartmann@t-online.de
Tel: (+49) 6071 1528
c/o Deutsche Telekom AG
Tel. (+49)6151 834965, Fax (+49) 6151 834284
The authors thank the other members of the ETSI TE1 VEMMI Working
Group for their comments:
- Michael Blaschitz (michael.blaschitz@etsi.fr)
- Agnelo Fernandes (agnelo@telepac.pt)
- Daniel Allonsius (daniel.allonsius@is.belgacom.be)
- Stefaan Herrebout (Stefaan.Herrebout@mail.interpac.be)
- Francisca Oliva (oliva@tid.es)
- Herwart Wermescher (Herwart.Wermescher@infonova.telecom.at)
11) References:
[1] "Enhanced Man-Machine Interface for Videotex and
Multimedia/Hypermedia Information Retrieval Services (VEMMI
revision 1)", ETS 300 709 standard (European Telecommunications
Standards Institute), September 1996.
This document is available on the Web in HTML format: see
http://www.etsi.fr/ecs/projects/vemmi/vemmi.htm
[2] "Enhanced Man-Machine Interface for Videotex and Other
Information Retrieval Services (VEMMI)", ITU-T T.107 standard
(International Telecommunications Union), March 1995.
[3] "Videotex Enhanced Man-Machine Interface service (VEMMI)",
ETS 300 382 standard (European Telecommunications Standards
Institute), February 1995.
[4] Fielding, R., "Relative Uniform Resource Locators", RFC 1808, UC
Irvine, June 1995.
[5] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform Resource
Locators (URL)", RFC 1738, December 1994.
[6] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700,
October 1994.
Mavrakis, et. al. Standards Track [Page 10]
RFC 2122 VEMMI URL Specification March 1997
[7] Mavrakis, D., "VEMMI and Internet", TD 44, ETSI TE1 plenary
meeting in Brussels, October 20, 1995.
[8] Berners-Lee, T., Fielding, R., and H. Frystyk: "Hypertext Transfer
Protocol - HTTP/1.0", RFC 1945, MIT/LCS, UC Irvine, May 1996.
[9] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., and T.
Berners-Lee, Transfer Protocol - HTTP/1.1", RFC 2068, UC Irvine,
January 1997.
[10] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four Registration Procedures", BCP
13, RFC 2048, November 1996.
[11] Masinter, L., Zigmond, D., and H. Alvestrand, "Guidelines and
Process for new URL Schemes", Work in Progress.
[12] Berners-Lee, T., and D. Connolly, "Hypertext Markup Language
Specification - 2.0", RFC 1866, MIT/LCS, November 1995.
[13] "Port Numbers",
ftp://venera.isi.edu/in-notes/iana/assignments/port-numbers
[14] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource
Locators (URL)", Work in Progress.
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