RFC 1614
Network Access to Multimedia Information
Pages: 79
Informational
Informational
Part 2 of 3 – Pages 24 to 54
3. Existing Systems This chapter describes some existing distributed information systems in sufficient detail to reveal how they handle multimedia data, and analyses how well they meet the requirements outlined in the preceding chapter. 3.1. Gopher The Internet Gopher is a distributed document delivery service. It allows a neophyte user to access various types of data residing on multiple hosts in a seamless fashion. This is accomplished by presenting the user with a hierarchical arrangement of nodes and by using a client-server communications model. The Gopher server accepts simple queries, and responds by sending the client a node (usually called a document in this context). Client software is available for a large number of systems, including: o UNIX (character terminals) o X windows o Apple Macintosh o MS DOS
o NeXT
o VM/CMS
o VMS
o OS/2
o MVS/XA
Servers are available for systems such as:
o UNIX
o VMS
o Apple Macintosh
o VM/CMS
o MVS
o MS DOS
Gopher was developed at the University of Minnesota.
Gopher User Image
A Gopher client offers an interface into "gopherspace", which appears
to the user as a hierarchy of menus and document nodes, similar in
some ways to a file system hierarchy of directories and files.
Selecting an entry from a menu node causes a further menu to appear,
or causes a document to be retrieved and displayed.
As well as "ordinary" document nodes, Gopher has "search nodes" when
one of these is selected from a menu, the user is prompted for one or
more words to search on. The result of the search is a "virtual"
menu, containing entries for document nodes (within some subset of
gopherspace) which match the search. A special type of Gopher search
server called "veronica" provides access to a database of all
directory nodes in gopherspace. This allows a user to construct a
virtual menu of all Gopher menu items containing a particular word.
WAIS databases may also be located at Gopher search nodes, since some
Gopher servers understand the format of WAIS index files.
Gopher Protocol Gopher uses a client-server paradigm. The Gopher protocol runs over a reliable data stream service, typically TCP, and is fully defined in RFC 1436. The following paragraphs give an overview which is sufficient for understanding how multimedia data is handled in Gopher. A Gopher client opens a TCP connection to a Gopher server (defined by machine name and TCP port number), and sends a line of text known as the "selector" to request information from the server. The server responds with a block of data, and then closes the connection. No state is retained by the server. A null (empty) selector tells the Gopher server to return its "root" menu node, containing pointers to other information in gopherspace. A menu is returned from a Gopher server as a sequence of lines of text, each corresponding to one entry in the menu. Each line (which is sometimes called a "Gopher reference") contains the following data, which can be used by the client software to retrieve and display the corresponding node in gopherspace. o A single character which identifies the type of the node. Possible values of this type ID are given below. o A human-readable string which is used by the client software when it displays the menu entry to the user. o The selector which should be used by client software to retrieve the node. It is treated as opaque by the client software. o The domain name of the host on which the node is held. o The port number to use for the TCP connection. A document node is sent by a Gopher server simply as lines of text terminated by a dot on a line by itself, or as raw binary data, with the end of the data indicated by the server closing the TCP connection. The choice depends on the type of node. The currently-defined type IDs are as follows: 0 Node is a file. 1 Node is a directory. 2 Node is a CSO phone book server.
3 Error.
4 Node is a BinHexed Macintosh file.
5 Node is DOS binary archive of some sort.
6 Node is a UNIX uuencoded file.
7 Node is a search server.
8 Node points to a text-based telnet session.
9 Node is a binary file.
T Node points to a TN3270 connection.
Some experimental IDs are also in use:
s Node contains -law sound data.
g Node contains GIF data.
M Node contains MIME data.
h Node contains HTML data.
I Node contains image data of some kind.
i In-line text type.
The process for defining new data types and corresponding IDs is not
clear.
Gopher+ Protocol
The Gopher+ protocol is an extension of the Gopher protocol. Gopher+
is defined informally in [4]. It is designed to be downwards
compatible with the original protocol, so that old Gopher clients may
access Gopher+ servers (without being able to take advantage of the
new facilities), and Gopher+ clients may access old Gopher servers.
Gopher+ is still at the experimental stage, and is liable to change.
The most important new feature is the introduction of "attributes"
associated with individual nodes. The client may retrieve the
attributes of a node instead of the node contents. Attributes
defined so far include:
INFO Contains the Gopher reference of the node.
Mandatory.
ADMIN Contains administrative information, including
the mail address of the server administrator and
the last-modified date of the node. Mandatory.
VIEWS Contains a list of one or more "view
descriptors", each of which describes an
alternate view of the node. For instance, an
image node may contain a TIFF view, a GIF view,
a JPEG view, etc. The client software (or the
user) may choose which view to retrieve. The
size of the view is also (optionally) available
in this attribute. The Gopher+ Attribute
Registry (see below) defines the permitted view
types.
ABSTRACT This attribute contains a short description of
the item. It may also include a Gopher
reference to a longer abstract, held in a
separate Gopher node.
ASK This attribute is used for the interactive query
extension. The interactive query facility in
Gopher+ is used to obtain information from a
user before retrieving the contents of a node.
The client fetches the ASK attribute, which
contains a list of questions for the user. His
or her responses to those questions are sent
along with the selector to the server, which
then returns the contents of the node. This
facility could be used as a very simple way of
querying a database, for instance. Using the
interactive query facility to supply a password
for access control purposes is not a good idea -
there are too many opportunities for
masquerading.
The University of Minnesota maintains a registry of Gopher+ attribute
types. For the VIEWS attribute, the registry contains a list of
permitted view types. Note that these view types have a similar
function to the type identifier described in the preceding section.
The general format of a Gopher+ view descriptor is:
xxx/yyy zzz: <nnnK>
where xxx is a general type-of-information advisory, yyy is what information format you need understand to interpret this information, zzz is a language advisory (coded using POSIX definitions), and nnn is the approximate size in bytes. Possible values for xxx include text, file, image, audio, video, terminal. (It now appears that the University of Minnesota Gopher Team accepts the need to be consistent in the use of type/encoding attributes with the MIME specification. The Gopher+ Type Registry may thus eventually disappear, together with the set of xxx/yyy values it currently contains.) No view descriptors for directory nodes are currently registered. In order to make use of the information available in attributes, it is necessary to fetch the attributes before fetching the contents of a node. Gopher+ provides a way of fetching the attributes for each entry in a menu at the same time as the menu is retrieved. This saves having to establish two successive TCP connections to fetch a single document, at the expense of some additional client software complexity. Gopher Publishing The procedure for making data available using the Unix Gopher server "gopherd" is very straightforward. The hierarchical nature of the Unix file system closely matches the Gopher concept of menus and documents. The gopherd program exploits this - Unix directories are represented as Gopher menu nodes, and Unix files as Gopher document nodes. The names of directories and files are the entries in Gopher menus. This can lead to awkward file names containing spaces, so gopherd provides an aliasing mechanism (the \.cap directory) to get round this. To represent menu entries pointing to Gopher nodes on other servers, special "link" files (starting with a dot) are used. The type ID for a document node is determined from the extension of its Unix filename. If a client requests a file containing a shell script, the script is executed and the output returned to the client. The Gopher+ version of gopherd is similar, but the .cap directory is replaced by a configuration file gopherd.conf. This file is used to specify administration attributes, and the mapping between filename extensions and view descriptors. Some limited access control (based on the client's IP address/domain name) is also provided by the Gopher+ version of gopherd.
Published Non-text Data There is already some useful non-text data published on Gopher almost exclusively image data. See for example the Vatican Library Exhibition at the University of Virginia Library, the ArchiGopher at the University of Michigan, the weather machine at the University of Illinois. Some of these are described in the User Requirements chapter of this report. There seem to be rather fewer sound archives in gopherspace, but interested users may access the Edinburgh University Computing Service Gopher server on gopher.ed.ac.uk, where the Testing Area contains 20 or 30 short audio files in Sun audio format. Note - the availability of this archive is not guaranteed. Advantages The main factor in favour of Gopher is its widespread penetration. There are over 1000 Gopher servers world-wide. This popularity is due in part to the ease of setting up a Gopher server and making information available on it, particularly on a Unix platform. Limitations It is unfortunate that the relatively well-defined MIME types were not adopted in Gopher+. As mentioned above, this may yet happen, although there appear to be reasons for keeping the set of MIME types small whereas Gopher requires a wide range of types to offer to clients. The latest word is that the MIME registry will be expanded to include the types which the Gopher+ developers want. Gopher is inflexibly hierarchical in nature. Hypertext or hypermedia it is not - links to other nodes from within document nodes are not possible. There is a suggestion in the Gopher+ specification that alternate views of directory nodes could be used to provide some kind of hypermedia capability, but this does not yet exist, and it is unlikely that it could be made to work as easily as the WWW hypertext model. There is no access control at the user level - anyone can retrieve anything on a Gopher server. There is no provision for charging for information. 3.2. Wide Area Information Server The Wide Area Information Server (WAIS) system allows users to search for and retrieve information from databases anywhere on the Internet. WAIS uses a client-server paradigm, and client and server software is
available for a wide range of platforms. Client applications are
able to retrieve text or other media documents stored on the servers,
by specifying keywords. The server software searches a full-text
index of the documents, and returns a list of documents containing
the keywords (ranked according to a heuristic algorithm). The client
may then request the server to send a copy of any of the documents
found. Relevant documents can be fed back to a server to refine the
search. Successful searches can be automatically re-run, to alert
the user when new information becomes available.
WAIS was developed by Thinking Machines Corporation of Cambridge,
Massachusetts, in collaboration with Apple Computer Inc., Dow Jones
and company, and KPMG Peat Marwick. The WAIS software has been made
freely available; however Thinking Machines has announced that they
will stop support for their publicly-distributed WAIS as of version
8b5.1. Future support and development of the publicly-distributed
WAIS has been taken over by CNIDR (Clearinghouse for Networked
Information Discovery and Retrieval) in the USA. Future CNIDR
releases will be called FreeWAIS. A new company, WAIS Inc, has been
formed by Thinking Machines to take over commercial exploitation of
the Thinking Machines WAIS software.
WAIS server software is available for the following platforms:
o UNIX
o VAX/VMS
Client software is available for the following platforms:
o UNIX (versions for X, Motif, Open Look, Sun View)
o NeXT
o Macintosh
o MS DOS
o MS Windows
o VAX/VMS
There are currently over 400 WAIS databases available on the
Internet. WAIS is also the basis of some commercial information
services on private networks.
WAIS User Image In order to ask a question, the user must first select one or more databases in which to look for the answer. (The list of all available databases is available from a number of well-known sites.) The next step is to enter one or more keywords as the basis of the search. The search will return a list of documents (the "result set") which contain any of the keywords. Each document is given a ranking (a number between 1 and 1000) which indicates how relevant to the user's question the server believes the document to be. The size of each document is also shown in the list. The user may limit the size of the result set - the default limit is typically 40 documents. The user may then choose to retrieve and display one or more documents from the list. Alternatively, he or she may designate one or more documents in the list as "relevant", and perform another search to find "more documents like this". This is called "relevance feedback". The user may retrieve general information about the database, and may examine the catalogue of all documents in the database. There is also a "database of databases", which may be searched to identify WAIS databases which may be relevant to a subject. WAIS Protocol The user interface (client) talks to the server using an extended version of a standard ANSI protocol called Z39.50. This is now aligned with the ISO SR (Search and Retrieval) protocol for bibliographic (library) applications, which is part of OSI. The present WAIS protocol does not utilise a full OSI stack - APDUs are transferred directly over a TCP/IP connection. The WAIS protocol is described in [5]. WAIS does not, at this time, implement the full Z39.50-1992 specification - in particular, WAIS does not permit Boolean searches (e.g., "find all documents containing 'chalk' and 'cheese' but not 'green'"). However, Boolean search capability is being added to the FreeWAIS implementation. There are facilities in the Z39.50 protocol for access control and charging, but these are not currently implemented in WAIS. The WAIS extensions to Z39.50 are mainly to provide the relevance feedback capability. Note that the Z39.50 protocol is not stateless - the result set may in some circumstances be retained by the server for the user to further refine or refer to. However, the subset of Z39.50 used by
current WAIS implementations mean that server implementations may be
stateless.
Document type is determined by the server from information in the
database index (see below), and is sent to the client as part of the
result set.
WAIS Publishing
The first step in preparing data for publishing in a WAIS database is
to use the 'waisindex' utility. This takes a set of text files, and
produces an index file which contains an occurrence list of words of
three or more letters in every file. This index file is used by the
WAIS server software to resolve search requests from clients.
The 'waisindex' utility indexes files in a wide range of text
formats, as well as postscript and image files in various encodings
(only the file name is indexed for image files). Some of the text
formats involve a file as being treated as a collection of documents
for the purposes of WAIS access. Note that there appears to be no
formal "registry of types" - just whatever the waisindex program
supports. There is no distinction between media type and encoding
format.
Published Non-text Data
There is relatively little non-text data available in WAIS databases.
o URL=wais://quake.think.com:210/CM-images is a database of
TIFF images from the Connection Machine.
o URL=wais://mpcc3.rpms.ac.uk:210/home/images/pathology/RPMS-
pathology is a database of histo-pathological images and
documentation on mammalian endocrine tissue.
o URL=wais://starhawk.jpl.nasa.gov:210/pio contains GIF images
from NASA planetary probe missions, together with their
captions. The presence of the caption index information
makes it difficult to construct a search which returns
images in the result set increasing the maximum result set
size may help.
Advantages
WAIS is ideally suited for its intended purpose of searching
databases of textual information on the basis of keywords. It
appears to have the potential to satisfy the requirements of some of
the "database" category of applications mentioned in Chapter 1.
Limitations WAIS is not (and does not pretend to be) a general-purpose information system, as Gopher and WWW are. WAIS does not have hyperlinking, and offers a purely flat structure. A limitation which is particularly apparent is the way that the current version of FreeWAIS indexes non-text files - using only the filename! However, it does seem that simply changing the indexing program to allow a list of keywords to be attached to non-text files would suffice to allow sensible indexing of non-text data. The commercial (WAIS Inc) version of WAIS allows several files to be associated together for indexing and retrieval purposes. Furthermode, the UCSF Centre for Knowlege Management is modifying the FreeWAIS code to support the indexing of multiple content types. The document returned by WAIS will be an HTML document containing pointers to the multimedia data. Contact dcmartin@library.ucsf.edu for further information. WAIS is not a fully-featured query/response protocol such as SQL. It has no concept of fields, or numeric data types. It appears to be impossible to retrieve a document from its catalogue entry in many of the existing databases. 3.3. World-Wide Web The World-Wide Web project (also known as WWW or W3), started and driven by CERN, is a large-scale distributed hypertext system. It uses the standard client-server paradigm, with client "browser" software responsible for fetching and displaying data. Originally aimed at the High Energy Physics community, it has spread to other areas. Browser software is available for a large number of systems including: o Line-mode dumb terminal. o Terminal with Curses support o Macintosh o X/Motif o X11 o PC/MS Windows
o NeXT
There is server software available for:
o VM mainframes.
o UNIX
o Macintosh
o VMS
WWW User Image
The WWW world consists of nodes (usually called documents) and links.
Links are connections between documents: to follow a link, a reader
clicks with a mouse on a word in the source document, which causes
the linked-to document to be retrieved and displayed. (On systems
without a mouse, the user types a number instead.)
Indexes are special documents which, rather than being read, may be
searched. To search an index, a reader supplies keywords (or other
search criteria). The result of a search is a "virtual" document
containing links to the documents found. All documents, whether
real, virtual or indexes, look similar to the reader.
The WWW addressing mechanism means that an interface to Gopher and
anonymous FTP information sources may be established, in a way which
is transparent to the user. Thus, the whole of gopherspace is part
of the Web. Transparent gateways to other systems, including Hyper-G
and WAIS, are also available.
URL
All nodes on the Web are addressed using the "Universal [or Uniform]
Resource Locator" (URL) syntax, defined in [6]. This is an Internet
Draft produced by the IETF URL Working Group.
A URL is a name for an object (which may be a document or an index)
on the Internet. It has the general form:
<scheme> : <path> [ # <anchorid> ]
The <scheme> identifies an access protocol or method for the object.
Some of the schemes are HTTP (the native WWW protocol), anonymous
FTP, Andrew file system, news, WAIS, Gopher. The <path> component
locates the document in a way significant for the access method.
Thus for instance for anonymous FTP, the path includes the fully qualified domain name of the host on which the document resides, and the directory and file name under which it may be found. For some schemes, the <path> may include a search string (or combination of strings) which is used to address a "virtual" object formed by searching an index of some kind. The HTTP, WAIS and Gopher schemes can use search strings, which usually follow the rest of the path, separated from it by a ?. The optional <anchorid> is used for addressing within an object. Its interpretation is not defined in the URL specification. "Partial" URLs may be specified. These are used within a document on the Web to refer to another "nearby" document - for instance to a document in another file on the same machine. Certain parts of the URL (e.g., the scheme and machine name) may be omitted, according to well-defined rules. This makes it much easier to move groups of documents around, while maintaining the links within and between them. A URL locates one and only one object on the Internet. However, more than one URL may point to the same object. Given two URLs, it is not in general possible to determine whether they refer to the same object. Furthermore, there is no guarantee that a single URL will refer to the same object at different times (the object may change incrementally, or it may be completely replaced with something different, or it may indeed be removed). HTTP HTTP (HyperText Transfer Protocol) is the protocol employed between server and client. It is defined in [7]. The protocol is currently being revised (see the Future Developments section below), and will eventually be proposed as an Internet standard. The original protocol is extremely simple, and requires only a reliable connection-oriented transport service, typically TCP/IP. The client establishes a connection with the server, and sends a request containing the word GET, a space, and the partial URL of the node to be retrieved, terminated by CR LF. The server responds with the node contents, comprising a text document in the Hypertext Markup Language (HTML). The end of the contents is indicated by the server closing the connection.
HTML HTML (HyperText Markup Language) is the way in which text documents must be structured if they are to contain links to other documents. Non-HTML text documents may of course be made available on the Web, but they may not contain links to other documents (i.e., they are leaf nodes), and they will be displayed by browsers without formatting, probably using a fixed-width font. Like HTTP, HTML is also undergoing enhancement, but the original version is defined in [7], and is being submitted as an Internet draft. HTML is an application of SGML (Standard Generalized Markup Language). It defines a range of useful tags for indicating a node title, paragraph boundaries, headings of several different levels, highlighting, lists, etc. Anchors are represented using an <A> tag. For instance, here is an example of HTML containing an anchor: The HTTP protocol implements the WWW <A NAME=13 HREF="../../Administration/DataModel.html">data model</A> . The location of the anchor is the text "data model". It is a source anchor, with a target given by the URL in the HREF attribute, so the text would appear highlighted in some way in a client's window, to indicate that clicking on it would cause a hyperlink to be traversed. It is also a target anchor, with an anchor ID given by the NAME attribute. A source anchor referring to this target would specify #13 at the end of the node's URL. Traversing a hyperlink to this node would cause the entire node to be retrieved, but the target anchor text would be displayed in some emphasised way - for instance if the retrieved text is displayed in a scrolling window, it might be positioned such that the target anchor appears at the top of the window. Another attribute of the <A> element, TYPE, is also available, which is intended to describe the nature of the relationship modelled by the link. However, this is not in extensive use, and there appears to be no registry of the possible values of such types. Future Developments HTTP and HTML are currently being extended in a backward-compatible way to add multimedia facilities. [8] describes the HTTP2 protocol. The revised HTML is defined in [9]. Both documents are subject to change (and indeed the HTML2 specification has changed substantially during the preparation of this report).
The revised HTML contains many enhancements which are useful for
multimedia support. Some of the most relevant are listed below.
o "Universal Resource Numbers" are a proposed system for
unique, timeless identifiers of network-accessible files
presently being designed by IETF Working Groups. URNs must
be distinguished from URLs, which contain information
sufficient to locate the document. URNs may be allocated to
nodes and may be represented in source anchors. This saves
client software from retrieving a copy of something it
already has - allowing sensible caching of large video
clips, for instance. The disadvantage is that when
something is changed and given a new URN, the source anchors
of all links which point to it must be changed (and the URNs
of these documents must therefore be changed, and so on).
Therefore, it makes sense to allocate URNs only to very
large documents which change rarely, and not to the
documents which reference them.
o The title of a destination document may be included as
anattribute of a source anchor. This allows a client to
display the title to the user before or during retrieval,
and also allows data which does not itself contain a title
(e.g., image data) to be given one.
o There is provision for in-line non-text data (e.g., images,
video, graphics, mathematical equations), which appears in
the samewindow as the main textual material in the node.
o The concept of the relationship expressed by a hyperlink is
expanded. Both source and target anchors may contain
relation attributes which point forwards and backwards
respectively. Possible relationships include "is an index
for", "is a glossary for", "annotates", "is a reply to", "is
embedded in", "is presented with". The last two are useful
for multimedia - for instance, the "embed" relationship
could cause a retrieved image to be fetched and embedded in
the display of a text node, and the "present" relationship
could cause a sound clip to be automatically retrieved and
presented along with a text node.
The HTTP2 protocol maintains the same stateless
connect/request/response/close procedure as the current HTTP
protocol. Data is transferred in MIME-shaped messages, allowing all
MIME data formats (including HTML) to be used. As well as the GET
operation, HTTP2 has operations such as:
HEAD Fetch attribute information about a node
(including the media type and encoding)
CHECKOUT/CHECKIN/PUT/POST
These allow nodes to be checked out for updating
and checked back in again, and new nodes to be
created. New node data is supplied in MIME
shape with the request.
The request from the client can contain a list of formats which the
client is prepared to accept, user identification, authorisation
information (a placeholder at present), an account name to charge any
costs to, and identification of the source anchor of the hyperlink
through which the node was accessed.
The response from the server may contain a range of useful attributes
(e.g., date, cost, length - but only for non-text data). The server
may redirect the query, indicating a new URL to use instead. It may
also refuse the request because of authorisation failure or absence
of a charge account in the request.
The protocol also contains a mechanism which is designed to allow the
server to make an intelligent decision about the most appropriate
format in which to return data, based on information supplied in the
request by the client. This may for instance allow a powerful server
to store the uncompressed bitmap of an image, but to compress it on
request using an appropriate encoding, according to the decoding
capabilities announced by the client.
An HTTP2 server and client are currently under test. Some HTML2
features are already fitted to the XMosaic browser.
Mosaic
The Mosaic project, located at the US National Centre for
Supercomputing Applications (NCSA) at the University of Illinois, is
developing a networked information system intended for wide-area
distributed asynchronous collaboration and hypermedia-based
information discovery and retrieval. Mosaic, which is specifically
oriented towards scientific research workers, has adopted the World
Wide Web as the core of the system, and the first Mosaic software to
appear was the XMosaic WWW client for UNIX with X. Other clients of
similar functionality are under development for the Apple Macintosh
and the PC with Windows.
The capabilities of the XMosaic browser include:
o Support for NCSA's Data Management Facility (DMF) for
scientific data.
o Support for transferring data with other NCSA tools such
asCollage, using NCSA's Data Transfer Mechanism (DTM).
o The ability to "check out" documents for revision, and to
check them back in again.
o Local and remote annotation of Web documents.
Future planned functionality includes:
o In-line non-text data (in addition to images).
o Information space graphical representation and control.
o Hypermedia document editing.
o Information filtering.
NCSA intends to make the entire Mosaic system publicly available and
distributable.
The XMosaic browser was used extensively for finding and retrieving
information used to prepare this report.
Web Publishing
Making a web is as simple as writing a few SGML files which point to
your existing data. Making it public involves running the FTP or HTTP
daemon, and making at least one link into your web from another. In
fact, any file available by anonymous FTP can be immediately linked
into a web. The very small start-up effort is designed to allow small
contributions.
At the other end of the scale, large information providers may
provide an HTTP server with full text or keyword indexing. This may
allow access to a large existing database without changing the way
that database is managed. Such gateways have already been made into
Digital's VMS/Help, Technical University of Graz's "Hyper-G", and
Thinking Machine's WAIS systems.
There are a few editors which understand HTML - for instance on UNIX
and on the NeXT platform.
Published non-text data See the multimedia demo node on: http://hoohoo.ncsa.uiuc.edu:80/mosaic-docs/multimedia.html This contains links to images, sound, movies and postscript media types. The media type is determined by the filename extension in the URL specification of the target node. The (XMosaic) client uses this to invoke a separate program appropriate for displaying the media type, or in some cases it can be displayed embedded within the source document. The latter method uses an <IMG> tag, which is part of HTML2. Advantages WWW is a hypertext system and its underlying technology is thus richer than Gopher. The use of SGML, which is of increasing importance in hypermedia systems, allows a great deal of expressiveness and structure, and enables text to be presented in an attractive way. The facilities for multimedia data in the extended versions of HTTP and HTML are excellent. It also seems that QOS and management issues identified in Chapter 2 are to some degree catered for in these extensions. Limitations There is no indication in the source anchor of the media type of the destination node, or of its size (this has been ruled out on the argument that the information is likely to degrade with time). It is necessary to perform a HEAD request (in HTTP2) to deduce this. Link source anchors must be in text documents, so non-text nodes must be leaf nodes. However, with HTML2 using the <IMG> tag, an embedded bitmap may be used as a source anchor, and the position of the mouse click within the image is passed to the server, which can then choose to return a different document depending on where in the image the mouse was clicked. WWW is much less prevalent than Gopher, partly because of an (erroneous?) perception that setting up an HTTP server is more complex than setting up a Gopher server. There are only about 60 servers world-wide; however the growth in the use of WWW is much faster than the growth in the use of Gopher. The availability of sophisticated WWW clients such as XMosaic is fuelling this growth.
3.4. Evaluating Existing Tools This section compares the capabilities of the Gopher, WAIS and WorldWide Web systems (abbreviated as GWW) to the informal requirements defined in section 2.3. Platforms The table below gives the names of the most important client software for each of GWW on the three most important platforms of interest. WWW is the weakest, with clients for the Macintosh and the PC still under development. The main PC Gopher client is "PC Gopher III", which is a DOS program, not a Windows program. CLIENTS Gopher WAIS WWW Macintosh TurboGopher WAIStation (No name) (beta version available) PC with HGopher (two WAIS for Cello (beta Windows others also Windows, WAIS version available) Manager available), Mosaic (beta due 3Q93) UNIX with X Xgopher, XWAIS XMosaic XMosaic At present, multimedia support in most of these clients (where it exists) is limited to the invocation of external "viewer" programs for particular media types. The exception is XMosaic, which supports in-line images in WWW documents. Media Types The GWW tools can all handle multiple media types well. o Text is very well supported by all three tools. WWW offers facilities for displaying "richer" text, supporting headings, lists, emphasised text etc., in a standardised way. o Image data is also well supported, using either external viewers (e.g., the TurboGopher client software on a Macintosh might invoke the JPEGView program to display an image); or in-line display within a text document (WWW with XMosaic on UNIX).
o There is little direct support for application-specific
data, but most systems allow data of a nominated type to be
passed to an external viewer or editor program. This tends
to be a function of the client software rather than being
built in to the protocol or server. There has been
discussion in the WWW community about using TeX for
representing mathematical equations, and about providing
"panels" within a text document where a separate application
could render its application-specific data (or indeed any
data which can be represented spatially). This latter
suggestion fits well with the OLE (Object Linking and
Embedding) approach used in Microsoft Windows.
o Sound can be supported through the external "viewer"
concept. Some platforms don't have readily-available
"viewers" with "tape recorder"-style controls for replaying.
There is no single commonly-accepted sound encoding format.
o Video data can be handled using external viewers. MPEG and
QuickTime are the most common encodings.
One essential capability of a client/server protocol is the ability
for the client to determine the type of a node (and a list of
available encodings) before downloading it. WAIS and Gopher transfer
this information in the result set and menu respectively. WWW
clients currently determine this information either from analysing
the URL of a target node, or by the occurrence of the <IMG> tag. The
new WWW HTTP2 protocol allows the media type and encoding of a node
to be determined through a separate interaction with the server.
The GWW systems all use different methods for expressing type and
encoding. WAIS does not distinguish the encoding from the media
type. WWW is moving to the MIME type/encoding system. Gopher does
not distinguish type and encoding, but Gopher+ does, and is also
moving to the MIME type/encoding system.
Hyperlinks
Only the WWW system has hyperlinks. Source anchors may be text,
images, or points within an image. Target anchors may be entire
nodes of any media type, or points within (with HTTP2, portions of)
text nodes.
Gopher+ could potentially be enhanced to include hyperlinks, but
there seems to be no development effort going towards this - those
who need hyperlinking are using WWW.
Gopher menus can be constructed to allow alternative views of
gopherspace. For instance, a geographically-organised menu tree of
gopherspace is in place, but a parallel subject-based menu tree could
be added as an alternative way of access to the same data. (There
are in fact moves to set this up.) Since WWW offers a superset of
Gopher functionality, these comments also apply to the Web. In fact,
the Web already has a rudimentary subject tree.
In both Gopher and WWW, non-textual data may be used in different
information structures without having to maintain more than one copy.
Presentation
There is little support in GWW for controlling the presentation of
non-text data.
o Backdrops are not supported by GWW.
o Buttons are supported in a limited way - typically, a node
is retrieved by clicking on a highlighted text phrase, or on
an entry in a list. In XMosaic, bitmap images can be used
as buttons. However, there is no support for different
styles of button. Client software may have generic
navigation buttons (e.g., "Back", "Next", "Home") which are
always available and don't form part of a node.
o Synchronisation in space is not supported by GWW, except
that WWW supports contextual synchronisation of images using
the <IMG> tag.
o Synchronisation in time is not supported by GWW.
Searching
WAIS supports keyword searching, and is very well suited for that
task. The Gopher+ protocol could potentially support multimedia
database querying applications through the ASK attribute, but there
is as yet no server implementation which supports such database
applications. In the WWW project, there are ongoing discussions on
how best to extend HTML to cope with database query applications - an
<INPUT> tag has been suggested - but no consensus has yet emerged.
Both Gopher and WWW can make use of WAIS-type keyword searching:
either by incorporating WAIS code into the server (enabling WAIS
index files to be searched); or through WAIS gateways, which run
searches on remote WAIS servers in response to queries from non-WAIS
clients.
Interaction XMosaic allows users to make text (or on some platforms, audio) annotations to any text node. The annotations appear at the end of the text display.. They are held locally - other users of the node do not see the annotations (but a recently added facility allows globally-visible annotations held on an "annotation server"). Text annotations may include hyperlinks to other nodes (provided the user knows how to use HTML). Other clients do not provide such facilities. There is a move to add an "email" address notation to URL. This would allow WWW client software to invoke a mail program when a user selects an anchor with such a URL. There are plans to allow WWW users to delineate a rectangular area of interest within an image for use in an HTTP request. There is no support in GWW clients for interacting with sequences of images in the way described in section 2.3.6. Quality of Service The user expectations for responsiveness mentioned in section 2.3.7 are difficult to meet with currently-deployed wide-area network (or even LAN) technology, particularly for voluminous multimedia data. None of the GWW systems currently exploit the emerging isochronous data transfer capabilities of protocols such as RTP and technologies such as ATM. None of them make serious attempts to alleviate the problem in other ways (except for WWW, which defines some mechanisms in HTTP2 for format negotiation based on size and available bandwidth considerations). Management The following table shows the support for three key management facilities in the GWW systems. The first two facilities require support in the client/server protocol, the third requires support in the server, but depends on authentication being available. Gopher WAIS WWW Access control No No1 Yes, in and HTTP2 authentication
Charging support No No Yes, in
HTTP2
Monitoring for No No No
statistical and
assessment
purposes
Note:
1. "Access-control-facility" is a feature of Z39.50 which is not used
by the current WAIS implementations.
Scripting Requirements
None of the GWW systems have facilities for the execution of scripts
by the client, because of security issues (it would be too easy for a
malicious "trojan" script to be executed). Gopher and WWW servers
have the ability for a UNIX script to be run by the server, with the
script output returned to the client. Scripting as understood in the
context of stand-alone multimedia applications does not exist in GWW.
Bytestream Format
None of the three GWW systems use a bytestream format for
interchanging collections of material. There has been some talk
about setting up a system akin to the "Trickle" mail server, for
retrieving single document nodes from GWW using mail. Such a system
has been implemented for WWW.
Authoring tools
Gopher is sufficiently simple to set up that no special authoring
tools are required. WAIS requires only an indexing program (as
discussed in section 3.2) for preparing material for publication.
WWW, because it uses a sophisticated authoring language (HTML),
benefits from the availability of authoring tools. There are HTML
editors for UNIX (using the tk toolkit) and the NeXT system. There
are no authoring tools designed specifically for exploiting the
multimedia capabilities of WWW, mainly because these capabilities are
still evolving.
4. Research This section describes some current research projects in the area of distributed hypermedia information systems. 4.1. Hyper-G Hyper-G [10] is an ambitious distributed hypermedia research project at a number of institutes of the IIG (Institutes for Information- Processing Graz), the Computing and Information Services Centre of the Graz University of Technology, and the Austrian Computer Society. It is funded by the Austrian Ministry of Science. It combines concepts of hypermedia, information retrieval systems and documentation systems with aspects of communication and collaboration, and computer-supported teaching and learning. Unlike WWW, Hyper-G supports bi-directional links. This enables users to see which other documents reference the one they are using, and also allows the system to avoid dangling pointers when a linkedto document is deleted. Another difference from WWW is that links are kept separately from their source and target nodes, to allow easy linking of read-only documents and for ease of link maintenance. In addition to manually defined links, Hyper-G supports automatic static and dynamic (i.e., view-time) generation and maintenance of links. Hyper-G has a concept of generic "structures" - an additional layer of relationships imposed on (and orthogonal to) the web of documents and links. A document can be part of more than one structure, and structures may be hierarchically related. Types of structure include: o "Clusters" are a set of documents which are all presentedtogether. o "Collections" are unordered sets of documents or other structures, and can be used as query domains or to construct gopher-like menus. o "Paths" are ordered sets of documents or structures, which must be visited sequentially. One application of the structure concept is the provision of "guided tours" through the information space. In addition to hypernavigation, the collection hierarchy and guided tours, another strategy for interaction with the system is the use of database queries. Two kinds of query are supported: keyword searching in a user-defined list of databases; and collection
specific form-filling queries. In the latter case, the answer to the query may appear dynamically as the form is filled out. Four modes of user identification are supported: "identified", where a userid is publicly associated through name and address information with a particular individual; "semi-identified", where a userid is associated by the system with an individual, but the user is only known to other users through a pseudonym; "anonymously identified", where the userid is not associated by the system with any individual; and "anonymous", where there is no userid (or a generic userid such as "guest"). Possible operations in the system depend on the user's mode of identification. Users may access the system in any desired mode, and switch to other modes only when necessary. Hyper-G contains specific support for multilingual documents and document clusters. Users may specify an ordered list of preferred languages, for instance. There are plans to experiment with automatic translation programs. Integration of other, external, systems such as WWW into Hyper-G in a seamless manner is possible. Hyper-G is in use as a CWIS within Graz Technical University. Client software is available for UNIX workstations from DEC, HP, SGI, and SUN. The system is still in an experimental state, but it has been used by about 200 students as part of a course on the social impact of information technology. 4.2. Microcosm Microcosm [11] is an open hypermedia system developed at the University of Southampton. It is implemented on the PC under MS Windows, and versions for the Apple Macintosh and for UNIX with X are under development. Microcosm consists of a number of autonomous processes which communicate with each other by a message-passing system. Information about hyperlinks between documents is stored in a link database, or "linkbase", and is not stored in the documents themselves. This has the advantages that: o Links to and from read-only documents (perhaps stored on CD- ROM) are possible. o Documents need undergo no conversion process to be imported into the system - they can still be viewed and edited using the original application which created them, without the link information getting in the way.
o It is as easy to establish links to and from non-text
documents as text documents.
In Microcosm, the user interacts with a "viewer" program for a
particular media type. Such programs may be specifically written for
use with Microcosm (about 10 such viewers have been written for a
number of common media types and encodings); or they may be a program
adapted for use with Microcosm (the programmability of Microsoft Word
for Windows has allowed it to be so adapted); or it may even be a
program with no knowledge of Microcosm.
The user selects an object (e.g., a piece of text) in the viewer, and
requests Microcosm to perform an action with the object - typically
to follow a link to another document. This may involve executing
another viewer to display the target document.
Microcosm link source anchors may be specific (denoting a unique
point in a particular document), local (denoting any occurrence of a
particular object in a particular document) or generic (denoting any
occurrence of an object in any document). Target anchors may specify
specific objects within a document. Other link styles are
textretrieval links (looking up a full-text index , as WAIS does),
and relevance links to a set of documents using similar vocabulary to
the source document (again, similar to WAIS's relevance feedback).
Links may be created by readers as well as by authors. Dynamically
computed links may be added to the permanent linkbase for later use.
A history of link traversal is maintained, and "guided tours" may be
established through the system which allow the reader to stray from
and return to the tour.
Microcosm viewers operate by sending messages to the Microcosm
system. In MS Windows, these messages are transferred using DDE
(Dynamic Data Exchange); in the Apple Macintosh version Apple Events
are used, and sockets are used on UNIX. For viewers which are not
Microcosm aware, the user must transfer the selected object to the
system clipboard before being able to follow a link from it.
Networking support in Microcosm is currently under development.
Components of Microcosm may be distributed to multiple machines there
is not necessarily a concept of "client" and "server".
There are problems with the Microcosm approach, common to systems
which maintain link information separately from documents, and which
use external viewers.
o Documents move and change, thus invalidating links.
Microcosm datestamps links to help to detect (but not
correct) such problems.
o It is not always clear what links are available to be
followed from a document, since the viewer program is
unaware of the contents of the linkbase.
o It is not always possible to indicate the object within a
document which is the target anchor of a link. Many viewers
automatically show the start of the document (e.g., a word
processor), or perhaps the entire document (e.g., a picture
viewer). The user has no way of knowing which part of the
target document the link just followed points to.
Microcosm may be viewed as an integrating hypermedia framework - a
layer on top of a range of existing applications which enables
relationships between different documents to be established.
Microcosm is currently being "commercialised".
4.3. AthenaMuse 2
AthenaMuse 2 (AM2) is an ambitious distributed hypermedia authoring
and presentation system under development by the AthenaMuse Software
Consortium based at MIT. It is based on the earlier AM1 system
developed as part of MIT's Project Athena. The first version of AM2
is scheduled for January 1994, and will be "pre-commercial software",
with a fully-commercialised version due about 6 months later. Both
the educational and commercial sectors are the intended market. The
system will initially be based on X and UNIX workstations, but
PC/Windows will also be supported in a second phase. Apple Macintosh
support has a lower priority.
The specifications of AM2 are available in [12]. Some of the key
points are:
o AM2 will support import and export of application from and
tostandard forms. The project is watching standards such as
HyTime, MHEG and ODA.
o Several "application themes", or frequently-occurring
collections of functionality, are viewed as useful. These
are as follows:
Application Theme Interactive?
Presentation of multimedia data No
Exploration of a rich multimedia Yes
environment
Simulation of a real-world scenario Partially
Communication of real-time No
information to the user
Authoring Yes
Annotation of material Yes
o "Interface templates" allow a multimedia application to make
use of a common format for presenting a range of content.
This is similar to the "backdrop" concept mentioned in
section 2.3.4.
o A range of link types will be supported.
o Media content editors and interface/application editors for
structuring will be provided. A third class of editor, the
"hypermedia notebook", will allow readers to excerpt and
annotate media from AM2 applications.
The project is developing multimedia network services, including the
transmission of digital video, using a client-server paradigm.
4.4. CEC Research Programmes
Some of the research programmes sponsored by the Commission for the
European Community (CEC) contain apparently relevant projects. [1]
has further details of some of these projects.
RACE programme
The RACE programme is outlined in [13], which should be consulted for
further information about the projects described below. The RACE
programme targets the industrial, commercial and domestic sectors,
and results are not necessarily directly applicable to the research
and academic community. RACE project numbers are given.
RACE Phase I projects, which have mostly completed:
R1038 MCPR - Multimedia Communication, Processing and
Representation. This project developed a demonstrator
multimedia system with communications capability for travel
agents.
R1061 DIMPE - Distributed Integrated Multimedia Publishing
Environment. The project designed and implemented interim
services for compound document handling, and defined a
distributed publishing architecture.
R1078 European Museums Network. This project aimed to demonstrate
interactive navigation through a pool of multimedia museum
objects, using ISDN as the communications network.
RACE Phase II projects:
R2008 EuroBridge.
Aims to demonstrate multi-point multimedia applications
running over DQDB, FDDI and ATM test networks.
R2043 RAMA - Remote Access to Museum Archives
This project follows on from R1078.
R2060 CIO - Coordination, Implementation and Operation of
Multimedia Services.
One aspect of this project is JVTOS - a "Joint Viewing and
Teleoperation Service". This aims to integrate standard
multimedia applications running on a range of heterogeneous
machines into a cooperative working environment, allowing
individuals to view and interact with multimedia data on
colleague's machines.
ESPRIT Programme
The ESPRIT research programme is outlined in [14], which should be
consulted for further information about the projects listed below.
ESPRIT project numbers are given.
28 MULTOS - A Multimedia Filing System
This project, which ran from 1985 to 1990, developed a
client/server system for filing and retrieval of multimedia
documents using the ODA interchange format standard (ODIF).
5252 HYTEA - HyperText Authoring
This project, which runs from 1991 to 1994, aims to develop
a set of authoring tools for large and complex hypermedia
applications.
5398 SHAPE - Second Generation Hypermedia Application Project
This project is developing a portable software environment
comparable to a CASE tool intended to facilitate the
realisation of complex hypermedia applications.
5633 HYTECH - Hypertextual and Hypermedial Technical
Documentation This project, which ran from 1990-1991, was to
assess the feasibility of hypermedia technology and to
devise needed extensions to it in order to support
applications dealing with technical documentation
management.
6586 PEGASUS - Distributed Multimedia Operating System for the
1990s This project is aimed at the design of an operating
system architecture for scalable distributed multimedia
systems and the development of a validating prototype, the
design and implementation of a distributed complex-object
service and a global name service, the development of
mechanisms for the creation, communication and rendering of
fully digital multimedia documents in real time and in a
distributed fashion, and the design and implementation of an
application for the system: a digital TV director.
6606 IDOMENEUS - Information and Data on Open Media for Networks
of Users. This project, which started January 1993, brings
together workers in the database, information retrieval,
networking and hypermedia research communities in the
development of an "ultimate information machine". It "will
coordinate and improve European efforts in the development
of next-generation information environments capable of
maintaining and communicating a largely extended class of
information on an open set of media". Because of the close
match between the subject of the IDOMENEUS project and the
RARE WG-IMM, it is recommended that RARE establish a liaison
with this project.
4.5. Other
Some other research projects of less immediate relevance are listed
below. Some of these projects are described further in [1].
o Xanadu is a project to develop an "open, social hypermedia"
distributed database server, incorporating CSCW features.
It has been in existance for many years and has been funded
by a number of companies. The current status of this
project is not known, and although iminent availability of
alpha-test versions has been announced more than once, no
software has been delivered.
o CMIFed [15] is an editing and presentation environment for
portable hypermedia documents being developed at CWI,
Amsterdam, NL. It is based on the "Amsterdam Model" of
hypermedia [16], which is an extension of the Dexter
hypertext reference model incorporating "channels" for media
delivery and synchronisation constraints.
o Deja Vu [17] is a proposed "intelligent" distributed
hypermedia application framework. It is intended as a
vehicle for research in the areas of: hypermedia systems,
object-oriented programming, distributed logic programming,
and intelligent information systems. Proposed techniques
for use in the Deja Vu framework include "inferential
links", defined automatically according to predefined rules.
A scripting language for use both by information providers
and users is planned. This project is at a very early
(proposal) stage, and as yet relatively little software has
been developed. Deja Vu is intended principally as a
research framework rather than as a service tool.
o Demon is a project at Bellcore, US, investigating the
network requirements of near-term residential multimedia
services. The project is designing and implementing an
experimental application which serves the needs of casual
multimedia users.
o InfoNote is a distributed, multiuser hypermedia system from
Japan, implemented on a NEC EWS4800 running UNIX and X.
InfoNote has an editor which can create Japanese texts,
figures, and raster images. The same windows are used both
for editors and browsers. The functionality of the window
can be changed at any time if data is not write-protected.
o MADE - Multimedia Application Demonstration Environment - is
a project at British Telecom's research laboratory which
centres on the use of the developing MHEG standard to access
a multimedia object server. The server platform is a Sun
SPARCstation with an object-oriented database package
(ONTOS). Audio, video, text and graphical media types are
covered. The University of Kent is working on a sub-
project: "Multi-user Indexing in a Distributed Multimedia
Database".
o Zenith aimed to establish a set of principles to assist
designers and developers of object management systems
intended for distributed multimedia design environments.
The project implemented a prototype generalised multimedia
object management system.
(next page on part 3)