Network Working Group
Request for Comments: 2122
Category: Standards Track
D. Mavrakis
Monaco Telematique MC-TEL
H. Layec
K. Kartmann
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 A presentation of
   VEMMI can be found on

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:


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

   - 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.

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://
      In this case, the exchange between client and server will be as
      follow (the server requests are presented left, client answers
   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://
      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://
      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:  {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://;$USERDATA=smith;account=1234
      If no access check is done by the host, the dialog might be:
   service:        demo;$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.

  • 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://">start the interactive
        multimedia service</A>, otherwise
        <A HREF="">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, 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.


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:


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.


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.


Refer to paragraph 8.


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
           Tel: (+377) 9216 8860
           Fax: (+377) 9330 4545

Comments may also be addressed to:

           Mr. Herve Layec,
           ETSI STC TE1
           06921 SOPHIA ANTIPOLIS Cedex
           Tel: (+33) 2 99 12 73 01
           Fax: (+33) 2 99 38 49 61
           Mr. Kurt Kartmann
           Gabelsbergerstr. 2
           D-64807 DIEBURG
           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 (
      - Agnelo Fernandes (
      - Daniel Allonsius (
      - Stefaan Herrebout (
      - Francisca Oliva (
      - Herwart Wermescher (

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

[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.
   [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",

   [14] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource
        Locators (URL)", Work in Progress.