WebSocket

WebSocket is a technology providing for bi-directional, full-duplex communications channels, over a single Transmission Control Protocol (TCP) socket, designed to be implemented in web browsers and web servers but can be utilised by any client or server application. The WebSocket API is being standardized by the W3C and the WebSocket protocol is being standardized by the IETF. Since ordinary TCP connections to ports other than 80 are frequently blocked by administrators outside of home environments it can be used as a way to overcome these restrictions and provide similar functionality with some additional protocol overhead whilst multiplexing several WebSocket services over a single TCP port.

For the client side, WebSocket is implemented in Firefox 4, Google Chrome 4, and Safari 5.

WebSocket Protocol Handshake

To establish a WebSocket connection, the client sends a WebSocket handshake request, and the server sends a WebSocket handshake response, as shown in the following example:

Browser request to the server:

GET /demo HTTP/1.1
Host: example.com
Connection: Upgrade
Sec-WebSocket-Key2: 12998 5 Y3 1  .P00
Sec-WebSocket-Protocol: sample
Upgrade: WebSocket
Sec-WebSocket-Key1: 4 @1  46546xW%0l 1 5
Origin: http://example.com

^n:ds[4U

Server response:

HTTP/1.1 101 WebSocket Protocol Handshake
Upgrade: WebSocket
Connection: Upgrade
Sec-WebSocket-Origin: http://example.com
Sec-WebSocket-Location: ws://example.com/demo
Sec-WebSocket-Protocol: sample

8jKS'y:G*Co,Wxa-

The Sec-WebSocket-Key1 and Sec-WebSocket-Key2 fields and the 8 bytes after the fields are random tokens which the server uses to construct a 16-byte token at the end of its handshake to prove that it has read the client's handshake.

The handshake is constructed by concatenating the numbers from the first key, and dividing by the number of spaces. This is then repeated for the second key. The two resulting numbers are concatenated with each other, and with the last 8 bytes after the fields. The final result is an MD5 sum of the concatenated string.

The handshake looks like HTTP but actually isn't. It allows the server to interpret part of the handshake request as HTTP and then switch to WebSocket.

Once established, WebSocket data frames can be sent back and forth between the client and the server in full-duplex mode. Text frames can be sent full-duplex, in either direction at the same time. The data is minimally framed with just two bytes. Each frame starts with a 0x00 byte, ends with a 0xFF byte, and contains UTF-8 data in between. Binary frames are not supported yet in the API. WebSocket text frames use a terminator, while binary frames use a length prefix.

Proxy traversal

WebSocket protocol client implementations try to detect if the user agent is configured to use a proxy when connecting to destination host and port and, if it is, uses HTTP CONNECT method to set up a persistent tunnel.

While the WebSocket protocol itself is unaware of proxy servers and firewalls, it features an HTTP-compatible handshake so that HTTP servers can share their default HTTP and HTTPS ports (80 and 443) with a WebSocket gateway or server. The WebSocket protocol defines a ws:// and wss:// prefix to indicate a WebSocket and a WebSocket Secure connection, respectively. Both schemes use an HTTP upgrade mechanism to upgrade to the WebSocket protocol. Some proxy servers are harmless and work fine with WebSocket; others will prevent WebSocket from working correctly, causing the connection to fail. In some cases additional proxy server configuration may be required, and certain proxy servers may need to be upgraded to support WebSocket.

If unencrypted WebSocket traffic flows through an explicit or a transparent proxy server on its way to the WebSocket server, then, whether or not the proxy server behaves as it should, the connection is almost certainly bound to fail today (as WebSocket become more mainstream, proxy servers may become WebSocket aware). Therefore, unencrypted WebSocket connections should be used only in the simplest topologies.

If an encrypted WebSocket connection is used, then the use of Transport Layer Security (TLS) in the WebSocket Secure connection ensures that an HTTP CONNECT command is issued when the browser is configured to use an explicit proxy server. This sets up a tunnel, which provides low-level end-to-end TCP communication through the HTTP proxy, between the WebSocket Secure client and the WebSocket server. In the case of transparent proxy servers, the browser is unaware of the proxy server, so no HTTP CONNECT is sent. However, since the wire traffic is encrypted, intermediate transparent proxy servers may simply allow the encrypted traffic through, so there is a much better chance that the WebSocket connection will succeed if WebSocket Secure is used. Using encryption is not free of resource cost, but often provides the highest success rate. A good example of use of an encrypted Websocket is by CloudSigma routing VNC traffic to enable a VNC client in the web browser.

Unfortunately, the most recent update to the draft (version 76) breaks compatibility with reverse-proxies and gateways because the 8 bytes of data the client must send after the headers is not advertised in a Content-Length header, so the intermediates won't forward that data until the handshake completes. And since the handshake needs those 8 bytes to complete, the handshake never completes and deadlocks. In current state of affairs, it's not advisable to modify such intermediate components to support this non-standard HTTP behaviour because doing so would render the components vulnerable to HTTP smuggling attacks, since an attacker would just have to pretend trying to upgrade to the WebSocket protocol in a request to be able to send more data than the target plain HTTP server can parse, possibly bypassing some mandatory security filtering. It is not known if this recent breakage will be worked around in a new draft or not.

URL scheme

The WebSocket protocol specification defines two new URI schemes, ws: and wss:, for unencrypted and encrypted connections. Apart from the scheme name, the rest of the URI components are defined to use URI generic syntax.

See also

• Push technology
• Comet

External links

• The WebSocket protocol - Internet-Draft at IETF
• The WebSocket API - W3C draft specification of the API
• WebSockets.org - a web-site dedicated to WebSocket
• HTML5 Server-Push Technologies, Part 2 - explains WebSocket