When the OSI defined its model it included a transport layer which was supposed to handle end-to-end connections and address communication reliability. In the early days of the web HTTP sat at the application layer (layer 7) and rode atop TCP, its transport layer.

An interesting thing happened on the way to the 21st century; HTTP became an application transport layer. Many web applications today use HTTP to transport other application protocols such as JSON and SOAP and RSS.

Applications now "speak" using a variety of languages to communicate, but underlying them all is HTTP. This is not the same as tunneling a different application through port 80 simply because almost all HTTP traffic flows through that port and it is therefore likely to be open on the corporate firewall. Those applications that simply tunnel through port 80 use TCP and their own application layer protocols, they're essentially just pretending to be HTTP by using the same port to fool firewalls into allowing their traffic to pass unhindered.

No, this is different. This is the use of HTTP to wrap other application protocols and transport them. The web server interprets the HTTP and handles sessions and cookies and parameters, but another application is required to interpret the messages contained within because they represent the protocol of yet another application.

In today's world the availability of exponentially expanding collaboration and syndication applications, all requiring different applications, is driving the need for smarter application delivery solutions to ensure availability, reliability, and scalability. Simple layer 4 (TCP) load balancing is not enough, neither is load balancing based on layer 7 (HTTP). Load balancing requests based on TCP or HTTP doesn't address the need to distribute application requests because the app is no longer HTTP, it's something else entirely. HTTP has been relegated to the status of application transport protocol, and that means in order to intelligently deliver an application we have to dig even deeper than layer 7. We've got to get inside.

The problem is, of course, that there are no standards beyond HTTP. My JSON-based Web 2.0 application looks nothing like your SOAP-based Web 2.0 application. And yet a single solution must be able to adapt to those differences and provide the same level of scalability and reliability for me as it does you. It has to be extensible. It has to provide some mechanism for adding custom behavior and addressing the specific needs of application protocols that are unknown at the time the solution is created.

This is an important facet of application delivery that is often overlooked. Applications aren't about HTTP anymore, they're about undefined and unknowable protocols. An application delivery solution can't distribute application load across servers unless it can understand which application it's supposed to be managing. And because HTTP connections are artificially limited by browsers, multiple application protocols are using the same HTTP connections over which to exchange data. That means an application delivery solution has to be able to dig into the application protocol and figure out where that request should be directed, and how to treat it, and what policies to apply. Application delivery today is about the message, not the protocol, and the message is undefined until it's created by a developer.

There's a lot of traffic out there that's just HTTP, as it was conceived of and implemented years ago. But there's a growing amount of traffic out there that's more than HTTP, that's relegated this ubiquitous protocol to an application transport layer protocol and uses it as such to deliver custom applications that use protocols without RFCs, without standards bodies, without the W3C.

If your application delivery solution doesn't offer a way that easily allows you to dig into the real application protocols, but instead relegates you to making load balancing and routing decisions based solely on HTTP, you need to reconsider your solution.

HTTP is the de facto application transport protocol today, but because it's so often used this way we have to get smarter about how we load balance and distribute those messages riding on HTTP if we want to architect smarter, greener, more efficient architectures.

Imbibing: Coffee