W3C HTTP1995

Progress on HTTP-NG

Here's some quick notes on how HTTP-NG is coming along - We've now got a simple proof-of-concept implementation up and running, using a minimalist set of requests and responses, and the results have been quite encouraging. Even though the network connection between Bristol and Civillization was extremely congested, initial timings seemed to show that HTTP-NG was capable of using the entire path bandwidth available when multiple requests were used. Initial measurements seemed to show that for typical inlined-objects, HTTP 1.0 would only use around a 10th of the available bandwidth, and that parallel fetches degraded badly under congested conditions.

Further reading:

A longer description of the architecture

The Session Control Protocol

A Quick Introduction to ASN.1

Analysis of HTTP/1.0 performance problems

Original March 1995 Draft


HTTP-NG is designed to be a replacement for HTTP 1.0 offering much higher performance and adding some extra features needed for use in commercial applications. The protocol is designed to make it easy to implement the basic functionality needed for a simple browser whilst making the addition of more powerful features such as security and authentication much simpler than it is for HTTP.

HTTP-NG uses a different basic model to HTTP. HTTP sets up a new connection for every request, which causes a lot of severe performance problems both in the time taken for each transaction, and the load placed on both networks and servers.

To avoid these problems, HTTP-NG allows many different requests to be sent over a single connection. These requests are asynchronous - there's no need for the client to wait for a response before sending out a different request. The server can also respond to requests in any order it sees fit - it can even interweave the data from multiple objects, allowing several images to be transferred in "parallel".

To make these multiple data streams easy to work with, HTTP-NG sends all it's messages and data using a "session layer". This divides the connection up into lots of different channels. HTTP-NG sends all control messages (GET requests, meta-information etc) over a control channel. Each object is returned over in its own channel.

This separation of data and control information also makes redirection much more powerful - for example, if the object is a video the server can return the meta-information over the same connection, together with a URL pointing to a dedicated video transfer protocol that will fetch the data for the relevant object. This becomes much more important when working with multimedia aware networking technologies, such as ATM or RSVP.

Implementation Experience

There are several obvious ways of implementing HTTP-NG. The first approach is to use a simple synchronous RPC model. The client establishes a connection to the server, then sends out a request. The client then waits until it has received all the data corresponding to the request, then returns. This approach is very simple to implement, and corresponds quite closely to current practice in older HTTP clients. As a simple optimization, the implementation could send a number of requests before waiting for all their responses.

The second approach is to use an event based model. A simple dispatcher can be wrapped around the connection. Each request then adds a callback that should be called whenever the response to the request comes in. This model works well with event based browsers such as Arena and Netscape; it is also quite simple to implement.

The final approach is to use a multi-threaded implementation; each request and channel gets allocated its own thread, allowing multiple requests to be processed in parallel. This model works supremely well for servers, especially those serving as proxies. The proxy server can keep a cache of open connections to other proxies or to popular sites. Since each connection resides within a single process, requests that can't be satisfied from the local cache can easily be switched to the appropriate connection. This approach also allows efficient gatewaying of HTTP 1.0 requests (see next section). Transition

The best transition strategy for moving from HTTP 1.0 to HTTP-NG is through the use of intermediate proxy servers. This allows the existing base of servers and clients to continues operating as they are now, whilst still taking advantage of much of the performance enhancements in the new protocol.

The reason that this works is that most of the performance problems in HTTP 1.0 are caused by delays in the network. If proxy servers are placed close to older clients and servers, then these delays become significant. For example, if two servers are placed at either end of a transatlantic link, communicating with each other using HTTP-NG, but accepting and sending requests to and from other systems using HTTP 1.0, all the HTTP 1.0 delays would all occur within a continent, rather than spanning the intercontinental links. Further, a cacheing server can interpret HTML documents and pre-fetch any inlined objects before an HTTP client requests them.

Simon Spero, UNC Sunsite/EIT (ses@unc.edu)