HTML 4.0 Sourcebook
(Publisher: John Wiley & Sons, Inc.)
Author(s): Ian S. Graham
ISBN: 0471257249
Publication Date: 04/01/98

Mid URLs

Mid URLs reference specific mail or USENET messages, or can reference a specific body part of a specific message. Thus, mid URLs can be thought of as a generalization of cid URLs, which reference parts within a given message. A mid URL takes one of the following two forms:

mid:message-id
mid:message-id/content-id

The first form references an entire message (identified by the Message-ID), while the second form references a particular part (identified by the Content-ID) of a specific multipart message (identified by the Message-ID). Here the message-id and content-id strings are the URL-encoded versions of the appropriate Message-ID and Content-ID headers, respectively, as defined by the MIME and USENET message protocols.

Mid URLs are supported by the Netscape Communicator mail, news, and HTML authoring clients, but only within the mail or news viewing client (Netscape Messenger). They are not understood by the Navigator 4 browser. However, the clients ignore the Message-ID portion of the URL and assume that the reference is internal within the displayed document. Essentially, then, Netscape treats mid URLs as equivalent to cid URLs. Mid URLs are not supported by Internet Explorer 4.

The following are two examples:

Mid:[email protected]
mid:[email protected]/[email protected]

An author would only rarely type a mid URL, as they are typically generated by the tool that assembles a mail message or newsgroup posting.

Mid URLs are related to cid (Content Identifier) URLs, discussed earlier in this chapter.

News URLs

News URLs reference USENET newsgroups or individual USENET news articles. There are several ways to compose news URLs. Particular newsgroups are specified using the form

news: news.group.name

where news.group.name is the name of a particular newsgroup. The special form

news:*

references all available newsgroups. Note that this does not specify an NNTP server from which news can be accessed. This must be specified elsewhere, in a browser-specific manner. Most browsers let the user set the name of the news server using a pull-down menu, while on UNIX systems it is set by defining the server domain name in an NNTPSERVER environment variable.

Referencing Particular News Articles

An alternative form is used to request particular news articles. The form is

news: [email protected]

where message_id is the unique ID associated with a particular article originating from the machine domain.name.edu (this is all discussed in gruesome detail in RFC 1036, referenced at the end of the chapter). The at (@) character is special in a news URL and indicates this alternative form of reference. This format is not generally useful, since most news servers delete articles after a few days or weeks, so that any referenced article is soon unavailable.

Referencing Both Server and Newsgroup

A third form, not part of the official URL standard but widely supported, references both the newsgroup and the server from which the articles should be retrieved or both the server and a particular article. The two forms are

<news:// int.domain.nam:port/news.group.name>
<news:// int.domain.nam:port/[email protected]>

where int.domain.nam is the domain name of the server to be contacted, :port is the optional port number (the default value is 119), news.group.name is the desired newsgroup, and message_id and domain.name.edu form the message ID for a specific message posted from the machine at domain.name.edu.

Snews URLs—Secure News

Many commercial Web browsers (in particular, those from Netscape) support snews URLs. Such URLs are composed—and behave—in exactly the same way as news URLs, the only difference being the default port number (563 for snews) and the fact that the connection between client and server is encrypted using the Secure Sockets Layer encryption technology. As a result, data can pass securely between client and server, without being intercepted and read by a third party. To access summary listings for a newsgroup accessible via a secure news server, the URL would be:

snews: news.group.name

The identity of the news server is in general set using browser Configure menus. SSL is discussed in more detail in Chapter 9.

NNTP URLs

In principle, the nntp URL scheme provides a way of explicitly referencing a news article from a particular NNTP news server. An example nntp URL might be

nntp://news.server.com/alt.rubber-chickens/[email protected]

which references article number [email protected], in the newsgroup alt.rubber-chickens, from the NNTP server running on the machine news.server.com (at the default port number 119). The NNTP protocol is described in RFC 977, should you want additional details.

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NOTE: Nntp Functionality Within News URLs

Essentially all the functionality of nntp URLs has been added to news URLs by allowing server domain name specification in the news URL, as described above. It is thus unlikely that nntp URLs will be widely implemented. At present, few browsers support this URL scheme.

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Prospero URLs

This scheme references resources accessible using the Prospero Directory Service, which acts as a sophisticated caching and proxying service for directory, file, and other data. Prospero URLs take the form

prospero:// int.domain.nam:port/string/stuff

which references a prospero server running at the indicated machine and port (the default port is 1525), the remaining portion of the string indicating the desired resource.

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NOTE: Prospero URLs Are Not Widely Supported

Very few Web browsers (i.e., practically none) support prospero URLs, so that this form is not discussed in detail here. Further information is found in the references at the end of the chapter.

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Telnet URLs

Telnet URLs reference a Telnet link to a remote machine. An example is

telnet:// int.domain.nam:port

where int.domain.nam is the machine to which a connection should be made, and the optional :port specifies the desired port (the default value is 32). A more general form is

telnet:// username:[email protected]:port/

to indicate the username and password that the user should employ (the :password is optional). The colon (:) and at (@) characters are special in a telnet URL, since they designate the different fields in this form. In general, the username and password information are not directly used by a browser to complete the connection. Rather, this information may be presented to users as a hint as to what they should do once the connection is made. Obviously you do not want to use this more general form if you want to keep a password secret!