VAST.Web Library

The VAST.Web library provides low-latency media streaming to browser clients over WebTransport (HTTP/3) with automatic WebSocket fallback. It is an ASP.NET Core extension that integrates with the VAST.Network.StreamingServer infrastructure.

Important

The WebTransport-based streaming protocol is a custom VASTreaming protocol and requires both a VASTreaming server and the VASTreaming JavaScript player on the client side. When the IETF Media over QUIC Transport (MoQ) specification becomes a standard, the library will provide a full MoQ implementation.

Overview

Feature	Description
WebTransport	Low-latency streaming over HTTP/3 QUIC
WebSocket Fallback	Automatic fallback for browsers without WebTransport support; such as Apple devices
Transcoding	On-the-fly video and audio transcoding for browser compatibility
Interactive Playback	Seek, pause, resume, and playback rate control for VOD streams
Keyframe-only Decoding	Client-requested keyframe-only mode for reduced bandwidth

Requirements

.NET: .NET 9.0 or later
Dependencies: VAST.Common
ASP.NET Core: Microsoft.AspNetCore.App framework reference

Supported Codecs

The WebTransport server automatically transcodes media to browser-compatible formats when needed.

Video

Codec	Description
H.264	Default output codec (Baseline profile)
H.265	Fallback if the client supports it

Audio

Codec	Description
AAC	Default output codec (ADTS format, 44.1 kHz stereo)

WebTransport Server

The WebTransportServer handles incoming WebTransport and WebSocket connections from browser clients.

Architecture

Note

WebTransportServer is not available as a standalone server. It operates as part of the VAST.Network.StreamingServer infrastructure and is instantiated by the StreamingServer automatically when enabled.

The diagrams below illustrate the media data flow inside the StreamingServer for the WebTransportServer server.

Live Streams

┌─────────────────────────────────────────────────────────────┐
│                 VAST.Network.StreamingServer                │
│ ┌─────────────────────────────────────────────────────────┐ │
│ │               VAST.Network.PublishingPoint              │ │
│ │                   /hls/stream1 (Live)                   │ │
│ │ ┌─────────────────────────────────────────────────────┐ │ │
│ │ │                  Live Media Source                  │ │ │
│ │ │           (Camera, File, Network Stream)            │ │ │
│ │ └─────────────────────────────────────────────────────┘ │ │
│ │                            ↓                            │ │
│ │ ┌─────────────────────────────────────────────────────┐ │ │
│ │ │       IWebTransportClientSink (internal class)      │ │ │
│ │ │          (Receives media, transcodes data)          │ │ │
│ │ └─────────────────────────────────────────────────────┘ │ │
│ └─────────────────────────────────────────────────────────┘ │
│                             ↓                               │
│ ┌─────────────────────────────────────────────────────────┐ │
│ │           WebTransportServer (internal class)           │ │
│ │              (client context management)                │ │
│ └─────────────────────────────────────────────────────────┘ │
│                             ↓                               │
│ ┌─────────────────────────────────────────────────────────┐ │
│ │                       HttpServer                        │ │
│ │            (HTTP request/response management,           │ │
│ │                 based on ASP.NET Core)                  │ │
│ └─────────────────────────────────────────────────────────┘ │
└─────────────────────────────────────────────────────────────┘
                              ↓
                Single HTTP Client (browser)

StreamingServer manages multiple live PublishingPoint instances. Each PublishingPoint has a single source object (IMediaSource) and a single IWebTransportClientSink (when WebTransport streaming is enabled). IWebTransportClientSink transcodes if necessary and prepares data for the connected client. The data prepared by IWebTransportClientSink is then served by WebTransportServer to the client via the HTTP server (ASP.NET Core).

VOD Streams

┌─────────────────────────────────────────────────────────────┐
│                 VAST.Network.StreamingServer                │
│ ┌─────────────────────────────────────────────────────────┐ │
│ │               VAST.Network.PublishingPoint              │ │
│ │                    /hls/movie1 (VOD)                    │ │
│ │ ┌─────────────────────────────────────────────────────┐ │ │
│ │ │                   VOD Media Source                  │ │ │
│ │ │                (File, Memory Stream)                │ │ │
│ │ └─────────────────────────────────────────────────────┘ │ │
│ │                            ↓                            │ │
│ │ ┌─────────────────────────────────────────────────────┐ │ │
│ │ │       IWebTransportClientSink (internal class)      │ │ │
│ │ │          (Receives media, transcodes data)          │ │ │
│ │ └─────────────────────────────────────────────────────┘ │ │
│ └─────────────────────────────────────────────────────────┘ │
│                             ↓                               │
│ ┌─────────────────────────────────────────────────────────┐ │
│ │           WebTransportServer (internal class)           │ │
│ │              (client context management)                │ │
│ └─────────────────────────────────────────────────────────┘ │
│                             ↓                               │
│ ┌─────────────────────────────────────────────────────────┐ │
│ │                       HttpServer                        │ │
│ │            (HTTP request/response management,           │ │
│ │                 based on ASP.NET Core)                  │ │
│ └─────────────────────────────────────────────────────────┘ │
└─────────────────────────────────────────────────────────────┘
                              ↓
             Single HTTP Client (browser, player)

StreamingServer manages multiple VOD PublishingPoint instances. Each PublishingPoint has a single source object (IMediaSource) and a single IWebTransportClientSink, and handles exactly one client because the source must maintain the current playback context (position, speed, etc.), which is unique to each connected client. IWebTransportClientSink transcodes if necessary and prepares data for the connected client. The data prepared by IWebTransportClientSink is then served by WebTransportServer to the client via the HTTP server (ASP.NET Core).

The VOD media source, typically IsoSource, uses shared metadata and indices cached in memory to skip parsing the file structure and perform only media read operations from disk.

Integration with StreamingServer

When used as part of StreamingServer, the WebTransport server shares the HTTP endpoints with other HTTP-based protocols:

var server = new VAST.Network.StreamingServer();

// Enable HTTP (required for WebTransport)
server.EnableHttp = true;

// Enable WebTransport
server.EnableWebTransport = true;
server.WebTransportServerParameters = new VAST.HTTP.WebTransportServerParameters
{
    HlsPath = "/wt",
};

server.Start();

Server Parameters

Configure server behavior using WebTransportServerParameters:

Parameter	Default	Description
WebTransportPath	null	Path relative to the root HTTP server path for resolving WebTransport requests; null means the root path is used

Transport Selection

The server automatically selects the transport based on the client request:

Transport	Protocol	Description
WebTransport	HTTP/3 (QUIC)	Primary transport; lowest latency via QUIC datagrams and streams; requires HTTPS
WebSocket	HTTP/1.1 or HTTP/2	Automatic fallback for browsers without WebTransport support

WebTransport is only available over HTTPS because TLS is a mandatory requirement of the HTTP/3 protocol. WebSocket connections can work over both HTTP and HTTPS.

WebTransport connections use the HTTP CONNECT method. WebSocket connections use the standard WebSocket upgrade handshake. Both transports use the same binary protocol and share the same publishing point infrastructure.

Troubleshooting

Common Issues

Issue	Cause	Solution
WebTransport not available	.NET version below 9.0	Upgrade to .NET 9.0 or later
Client falls back to WebSocket	Browser doesn't support WebTransport or HTTP/3 not configured	Ensure HTTP/3 is enabled on the server; check browser compatibility
Transcoding failure	Missing codec support on server	Verify encoder/decoder availability on the platform
Client timeout	No activity within 30 seconds	Ensure client sends keep-alive (Noop) packets
Negotiation timeout	Client didn't respond to media descriptor offer	Check client-side JavaScript player initialization

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