Windows Server 2012 : Support for open standards

WebSocket handshake

To establish a WebSocket connection, the client and server perform a “handshake” where they agree that they both understand the same version of WebSocket and the requested server resource supports WebSocket. The following client request and server response make up the handshake performed to establish a WebSocket connection.

The following is a sample WebSocket request from a client:

   GET /sampleapp HTTP/1.1
Host: contoso.com
Upgrade: websocket
Connection: Upgrade
Origin: http://contoso.com
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

IIS handling of WebSocket request

When the server evaluates this request, it notices that the Upgrade header is requesting that the connection be upgraded to a WebSocket connection. The server responds with an HTTP 101 response indicating that the protocol is being changed to use WebSocket.

IIS implements a native WebSocket module on the IIS request pipeline architecture, which applications can use to communicate over WebSocket. The IIS WebSocket module listens on the RQ_SEND_RESPONSE notification of the request pipeline.

On send response notification (before the response is returned to the client), if the HTTP status code of the response is 101, IIS calls into the Websocket.dll (the Win32 library in Windows Server 2012, which implements WebSocket framing). The WebSocket dll then computes a value for the Sec-WebSocket-Accept header based on the value of the Sec-WebSocket-Key from the request header.

These values are then set into the response headers. On the send call to HTTP, IIS also sets the HTTP_SEND_RESPONSE_FLAG_OPAQUE flag, which indicates that HTTP should go into opaque mode. This flag tells HTTP that the request and response from that point on will not be HTTP-compliant, and all subsequent bytes should be treated as an entity-body and appends the Sec-WebSocket-Accept header to the response.

The following is a sample response from a server:

   HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

The server response indicates to the client that it is switching to WebSocket and returns the result of the operation that it performed on the Sec-WebSocket-Key in the Sec-WebSocket-Accept header. The client uses this to confirm that the server properly understands WebSocket. This concludes the handshake.

Using the WebSocket connection

If the handshake is successful, applications can get a pointer to the IWebSocketContext interface from the IHttpContext of the request. The IWebSocketContext interface is stored in the Named Context containers of IHttpContext. Applications can query the named context container with the query key “websockets” to get the pointer to this interface.

Applications can then do WebSocket I/O through the APIs exposed by this interface. WriteFragment, ReadFragment, SendConnectionClose, GetCloseStatus, and CloseTcpConnection are the APIs implemented for Windows Server 2012.