Simply put, a trackback is a means of establishing links between webpages owned by two different people.  What is interesting about a trackback is that only one person creates the links, even though he controls only one of the webpages.

Here's a classic example.  I write a post about Elections Canada and their Investigators' Manual.  Jason Cherniak reads it and writes a rebuttal.  He links to my post, obviously, in order to give his readers context.  But because my post came first, there is no link from my post to his post, even though, as a matter of principle, I want people to read a wide range of opinions.

Now I could have noticed that there was traffic coming from Jason's blog, and set up a link later.  But I was linked by Bourque too, and the surge in traffic tends to wash out other traffic in the snapshot view.  Jason could have informed me of the post and asked for a link, but he's a busy guy, I'm a busy guy -- and then what if there are 20 bloggers posting responses and rebuttals?  I'd be swamped with requests for links to these other blogs.

The solution is the trackback, an idea first developed by Six Apart for their Movable Type blog.  Essentially, I create a space in which other webpage owners (usually, but not necessarily, other bloggers) can post a link back to their page from my post. 

Here's how it works.  I provide a special URI for you to reference.  It does not lead to a webpage, but it identifies this trackback space for a particular post.  To write into this space, you have to send a trackback ping.  The ping is a message with the following information:

• the trackback URI I provided for my post, the one that you wrote about
• the title of the webpage (usually a blog post) you wrote
• the URL of the webpage (usually a blog post) you wrote
• an excerpt from, or a description of, the webpage (usually a blog post) you wrote

What you might notice is the distinct lack of passwords in the message.  This is a problem with trackbacks.  Anyone can send a trackback ping.  Indeed, Person A can send a trackback ping to Person B's blog to create a trackback link to Person C's webpage, without either Person B or Person C being involved.

That lack of any sort of authentication is what made trackbacks so susceptible to spam.  A more recent variation of trackbacks is the pingback, which requires that an actual link exist from the new page to the original page for the ping to succeed.  Since trackbacks don't require this link, this makes pingbacks more secure.  But I can't set up pingbacks, at least not yet.

But despite the risk, I've set up trackbacks using Haloscan as the trackback manager, as opposed to my own blog software.  Since Haloscan is used by thousands of bloggers, it has developed a large table of known spam IPs that helps filter out bad trackbacks.

So how do you use trackbacks?  If you are a blogger, your blog software probably has a trackback ping utility built in.  You write your post referencing my post.  Then you go to my post and copy the trackback URI for my post:

It is important to remember that you don't want the actual permalink for my post, but this special URI.

Paste the trackback URI grabbed from my post into the appropriate trackback box of your blog post editor, and you're done.  The utility will grab information about your post, combine it with the trackback URI you grabbed from my post to create a trackback message, and then send me the trackback ping.  My blog software will process the ping, and add your post to the list of trackbacks.  Here is Jason's post as a trackback:

But trackbacks aren't just for blogs.  Any webpage that references my post could be a good candidate for a trackback.  But then how do you send a trackback ping if you don't use blog software?

There are webpages that provide you with a form into which you enter all the relevant information, and then it will send the ping.  Yes, this is another security hole, in that the trackpack specification does not make it necessary for the ping to originate from the same IP address as the webpage, but in this case, this oversight helps.  The O'Reilly people have a nice, straightforward trackback form:

Here is how the form looks just before submitting the trackback ping for Jason's post:

So whether you run a blog or some other website, and should you find reason to link to my blog, and you want people visiting my site to read what you have to say, feel free to set up a trackback.  I'll monitor usage for the next while, to see if spamming becomes too much to manage.

I hope we all benefit from the mutual links, bloggers and readers alike.

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