Httpalt

Httpalt is a term used in some discussions of web protocols to denote an alternative transport path for HTTP-like interactions. It is not an official standard and there is no single, formal definition. In practice, httpalt is used as a label for mechanisms that allow clients to access the same resource via an alternative protocol or transport while keeping the same resource URL semantics. Common motivations include reducing latency, improving throughput, or enabling features not available in standard HTTP, such as persistent connections or bidirectional streams.

Implementation patterns vary widely. A typical approach uses a discovery mechanism where a client learns an alternate endpoint or service by querying an initial API or through a header, DNS, or DNS-based service discovery. The server then provides a mapping or an alternate URL that the client can use for subsequent requests. The alternate path may use protocols like HTTP/2, HTTP/3, WebSocket, or a custom layer within TLS.

Challenges arise in the operation of httpalt. Caching proxies, intermediate gateways, security and cross-origin policies, TLS certificate validation across transports, and maintaining compatibility with caches and logs are common concerns. Interoperability with existing HTTP infrastructure can be complex, as intermediaries may treat the alternate path differently from standard HTTP traffic.

Relation to standards is limited. While mechanisms such as the Alt-Svc header hint at the possibility of alternate services, httpalt itself is not standardized and implementation details vary between projects. It remains a concept used in certain architectures rather than a universally adopted protocol.

See also: Alt-Svc, HTTP/2, HTTP/3, WebSocket.