Information Technology Reference
In-Depth Information
various application protocols. The most popular
application protocol in Internet is HTTP, by ex-
changing the messages of which between clients
and servers, over the TCP/IP infrastructure, WWW
is implemented.
On the client side, the HTTP protocol is imple-
mented by the so-called User Agents (User Agent
wikipedia). The most common Web User Agents
are the well-known Web browsers. A Web browser
is an application client which transmits HTTP
requests to Web Servers, receives the responses
and presents to the user the associated content.
A HTTP request to a server, normally, looks like:
is compatible with Mozilla/4.0 and MSIE 5.0
browsers and runs on a Windows ME platform.
Other common HTTP headers (IETF RFC 2616),
characterizing the client's environment, are:
•
Accept:
Identifies the file and content-
types that the client accepts
•
Accept-Charset:
Identifies the Character
sets that are acceptable
•
Accept-Language:
Identifies the accept-
able languages for response
•
Accept-Encoding:
Identifies the accept-
able encodings
•
Cookie:
Includes an HTTP cookie previ-
ously sent by the server
GET/images/logo.html.HTTP/1.1
•
X-Wap-Profile:
It may includes a refer-
ence to a full client device's capability
proile
Host:www.servername.gr
With this HTTP 1.1 request, the client requests
from a server named www.servername.gr to view
the file
logo.html
stored in the root path
/image/
logo.html
. The path
/image/logo.html,
which
identifies the requested resource in the server, is
called in general URI (Uniform Resource Identi-
fiers) (URI wikipedia). GET is the standardized
name of the HTTP message. In (IETF RFC 2616)
various messages are defined (e.g. the POST,
DELETE, HEAD) but GET is the most common.
Normally, when a user agent operates, it typi-
cally identifies itself, its application type, operating
system, software vendor, or software revision, by
submitting a characteristic identification string
to its operating peer. In the HTTP protocol, this
is transmitted in the header field “
User-Agent
”,
which belongs to the HTTP metadata accompany-
ing HTTP messages and characterizing clients. The
syntax of the
User Agent
header field looks like:
•
Profile:
It may includes the client's profile
or a reference to it
•
Options:
Includes additional information
for the client
Definitely, the information included in these
HTTP headers is only indicative since it does
not provide detailed information related to the
hardware, software and networking environ-
ment at the client-side. Such information is not
adequate to constitute the server-side aware of
the client's capabilities. However, although each
of the HTTP headers above contributes to a very
limited perspective of the execution context at
client-side, some of them, properly used, can offer
a full view to a service developer. In the following
section we will detail in the CC/PP and MPEG-21
mechanisms standardized to this end.
UAProf and CC/PP
User-Agent: (Mozilla/4.0compatible;
MSIE 5.0; Windows ME) Opera 5.11 [en]
UAProf (User Agent Profile) (UAProf specifica-
tion) is an XML document that contains informa-
tion about the user agent type and device capa-
bilities. It is a standard defined and maintained
With this example announcement, the client
announces to a server that its agent is the Eng-
lish version of the Opera 5.11 browser, which
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