1. The number of transactions to download an HTML file with inline
images should be minimized.
2. The client should have the option of downloading all, some or none
of the inline images. It should be able to request some or all
files with an "If-Modified-Since:" header.
3. Additions to the protocol should be simple to implement for
servers and browsers.
4. The statelessness of the server should be preserved.
5. New client / old server and old client / new server transactions
should work.
6. The server must be able to transmit the multiple files "on the
fly" without knowing their size in advance and without making a
copy of the whole package before transmission.
7. The server must have the option of returning some of the requested
files while denying access or reporting an error for others. In
particular, it must return a separate status header for each
requested file.
In order to achieve the second objective above a minimum of two
transactions will be required. In the first transaction the client
receives the base HTML document (this is a GET transaction and should
be identical to HTTP/1.0). The client is then in a position to decide
which inline images to request. It may want all, some, or none, as it
may have some cached or it may be incapable of displaying images.
The second transaction is an MGET transaction. The client lists the
URIs it wants each followed by an "Accepts:" header applicable to that
URI alone. The client can also provide an "If-Modified-Since:" header
for any of the requested files which should work like it does in
HTTP/1.0 The server returns the files with a packet transfer-content
encoding beginning each packet with the exact number (in ASCII) of
bytes in that packet and an empty packet (size -1) to indicate end of
file. Here is an example of a client server exchange.
C: GET /foo.html HTTP/2.0<CRLF>
Accept: text/html, text/plain<CRLF>
S: HTTP/2.0 200 Success<CRLF>
Content-Type: text/html<CRLF>
...<other headers>
<CRLF>
<sends file>
< closes connection>
[This was a GET request, identical to HTTP/1.0. The second request uses
MGET.]
C: MGET HTTP/2.0<CRLF>
URI: /images/bar1.gif<CRLF>
If-Modified-Since: Saturday, 29-Oct-94 20:04:01 GMT
Accept: image/gif, image/x-xbm<CRLF>
URI: /images/bar2.gif<CRLF>
If-Modified-Since: Saturday, 29-Oct-94 20:04:01 GMT
Accept: image/gif, image/x-xbm<CRLF>
URI: /images/bar3.gif<CRLF>
Accept: image/gif, image/x-xbm<CRLF>
URI: /images/bar4.gif<CRLF>
Accept: image/gif, image/x-xbm<CRLF>
<CRLF>
S: HTTP/2.0 200 Success<CRLF>
URI: /images/bar1.gif<CRLF>
Content-Type: image/gif<CRLF>
Content-Transfer-Encoding: packet<CRLF>
<CRLF>
8000<CRLF>
... 8000 bytes of image data first packet...
2235<CRLF>
... 2235 bytes of image data completing file...
-1<CRLF>
<CRLF>
HTTP/2.0 304 Not Modified<CRLF>
URI: /images/bar2.gif<CRLF>
Expires: Saturday, 29-Oct-95 20:04:01 GMT
<CRLF>
HTTP/2.0 403 Forbidden
URI: /images/bar3.gif<CRLF>
<CRLF>
HTTP/2.0 200 Success<CRLF>
URI: /images/bar4.gif<CRLF>
Content-Type: image/gif<CRLF>
Content-Transfer-Encoding: packet<CRLF>
<CRLF>
150213<CRLF>
... 150213 bytes of image data (complete file)...
-1<CRLF>
<CRLF>
S: <closes connection>
This seems to me to meet all the objectives listed above. Comments are
welcome.
John Franks Dept of Math. Northwestern University
john@math.nwu.edu