W3C home > Mailing lists > Public > w3c-wai-er-ig@w3.org > February 1999

RE: Betsie / Gateway comparison

From: Silas S. Brown <ssb22@cam.ac.uk>
Date: Fri, 26 Feb 1999 13:45:53 +0000
To: Wayne Myers-Education <wayne.myers@bbc.co.uk>
CC: w3c-wai-er-ig@w3.org
Message-Id: <E10GNaS-0005IF-01@green.csi.cam.ac.uk>
Hi, thanks for your message.  I agree there certainly is a limit to what 
a gateway can do (take those stupid pages that are 100% gif files, for 
example); my development of the access gateway has been largely steered 
by my own browsing - if I want to see page X, and the gateway won't do 
it, then I look at the HTML and try and work out a way of making it 
better.  Same for browsers - recently (for various stupid reasons I 
won't go into) I was forced to use NCSA Mosaic for a while, and it 
turned out it couldn't display in a large font unless it was headings, 
so I hacked out an option to make everything a heading.  It's not very 
good but it worked.

If Betsie is being used as a repair tool then it's probably not a good 
idea to implement lots of stuff like that - it encourages bad HTML.  One 
problem is that webmasters might stick Betsie up and assume that this 
somehow magically makes everything right.  One interesting extension 
would be to get it to log any obvious mistakes it finds and can't deal 
with (images without ALT, etc).  I won't get the gateway to do this 
because it has a slightly different purpose - it's supposed to help with
other people's sites, not your own.  (I did think of getting it to 
automatically send an email complaining about lack of ALT tags to 
webmaster@wherever, but I thought that this could easily get out of 
control.)

[TD instead of /A]
> And... yes, but it's not a common idiom, it's a common error. Mind you, it
> would be good if Betsie could fix that too.

Especially since it occurs several times on the BBC's site (or maybe 
it would be better to log these things).  I found out about it because
MetaCrawler uses it - it was incredibly unhelpful to have a page of 
search results all as one link.

> > Also, the link is to a graphics file, which Betsie is unaware of and 
> > tries to redirect through itself.
> 
> Um.. doh! Again! Hadn't thought of that. (Not that it happens often).

A very common thing is to do
<A HREF="bigfile.gif"><IMG SRC="smallfile.gif"></A>
(ie. click on the image for a larger view)

It's not just images - it's things like .tar.gz etc as well.  If you 
look at the top of tagstuff.c++, there is a list of extensions that the 
gateway avoids, if that's any help.  (// is a comment by the way)

> There's
> another bug you missed, in fact, which is that you can point Betsie at
> herself and she doesn't mind, and you can keep doing that until your browser
> crashes with an over-long URL. This has to be fixed too. (My strong
> suspicion is that you've already sorted that one out in yours.)

It's not too difficult - you make sure every page you generate contains 
a funny string (eg. a bunch of random letters in a comment), and if, 
when reading a page, you find that funny string, you give up.

In my gateway I made all my variables (eg. URL to look at) begin with 
ssb22A.  This makes it easy to submit forms through the gateway, since 
you can tack on the form parameters to my parameters and it will be OK.  
(In Betsie this is less of a problem since it only ever takes a URL 
anyway and no extra parameters, but if you want to introduce parameters 
then you'll have this problem.)  Then, when parsing a form, if I saw 
ssb22A, I'd give up.  This made sure the gateway couldn't go through 
itself.

Recently, however, I actually removed this feature.  The reason for this 
was someone here at Cambridge University putting up a web page which, 
when I looked at it though the gateway, caused the gateway to complain 
of a reference to itself.  I looked at the HTML and found this comment:

<!-- Stop Silas from reading my page -->

The obvious implication is that, if you use this method, it is possible 
for someone to write a page that deliberately excludes all gateway 
users.  I also had a problem when I wanted to get one instance of the 
gateway to fetch a page from another, in order to try and get around a 
routing loop in the UK's Joint Academic Network last weekend.

So I changed the program (although you probably still have the old 
version).  Now, instead of using ssb22A, it just uses A (although it 
still understands the old syntax so people's bookmarks still work), and 
if it finds something already beginning with A, it prefixes it with an 
"escape sequence" type thing.  Then, when you submit through the 
gateway, it removes one level of escape sequences.

So now it's possible to follow a gateway'd link and end up nested.  And 
it would be no good to just check to see if it's referring to itself, 
because you could get a gateway on machine A to go through one on 
machine B, which in turn goes back through A.  One thing you could do is 
check the user agent - I set my user agent to Access_gateway and I could 
easily throw an error message if someone tries to get pages from the 
gateway through itself (or another one).  But at the moment I haven't 
done anything, because this corner of the Internet is always having 
problems and I find the "forwarding around black holes" kind-of useful!  
(Could just do with one in France or Germany to avoid the transatlantic 
link that keeps going down....)

BTW One idea you might like is to add this in the head of all pages 
returned by Betsie:

<META NAME="robots" CONTENT="noindex,nofollow">

because there's absolutely no point in robots taking up your processor 
time.  Some robots will still follow through, most notably spam email 
collectors; have a look at spamhate.c++ to see how I deal with them.  
(Does anyone know if Mercator is a real browser, by the way?)

> In terms of multiple passes through the page, though, they're not countless,

Not yet!  Betsie's speed is fine at the moment; I'm just thinking of 
what would happen if you tried to implement all the options in the 
gateway in Perl.

I agree Perl encourages you to write this way, because of all of this 
regular expression stuff.  I tend to think state-based, ie. "If you see 
this, go into a state where you're ignoring stuff until you see this", 
and that way I can do all the removal in the same pass.  C++ does help 
because it lets you have objects - I make a Tag an object with methods, 
and this helps a lot with processing individual tags.

> Meanwhile, as pages get larger the Betsie version of the page seems to
> actually increase in download speed

Hmm, interesting!  Could this be because Betsie is removing some of the 
stuff (especially images)?  With my gateway I find it's usually slower 
than direct browsing (especially if the gateway is going through the 
proxy we're made to use, which is rather slow); I don't start sending 
stuff immediately because a lot of web servers don't support it - they 
collect all the stuff together and send it themselves, with 
Content-Length attached.  (And the server on my machine doesn't even do 
that - it expects you to work out the Content-Length, and if you don't, 
the browser hangs because it uses the keep-alive protocol.)  If the 
server's going to collect all the stuff then I might as well hang on to 
it myself, and this allows doing such things as erasing half-finished 
pages when an unrecoverable error happens.  (There's nothing worse than 
an error message deeply embedded in a page)

I find that the slowest part of the whole process (now I've optimised 
the program itself) is waiting for the remote computer to respond while 
getting the page.  But I do want to make sure my program adds as little 
overhead as possible.

[java]
> Um.. oh. It's quite possible that I'm wrong then, but I couldn't get it to
> work.

What URL did you test it on?  All I do is make sure the class file etc 
is specified as an absolute URL.

Regards

-- Silas S Brown, St John's College Cambridge UK http://epona.ucam.org/~ssb22/

"Better is the end afterward of a matter than its beginning" - Ecclesiastes
 7:8
Received on Friday, 26 February 1999 08:45:58 GMT

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