W3C home > Mailing lists > Public > whatwg@whatwg.org > July 2008

[whatwg] Web Sockets

From: Shannon <shannon@arc.net.au>
Date: Wed, 23 Jul 2008 16:17:55 +1000
Message-ID: <4886CD13.6020504@arc.net.au>
Philipp Serafin wrote:
>
>> Asynchronous: Requests and responses can be pipelined, meaning requests and
>> responses can be transmitted simultaneously and are queued.
>>     
>
>
> I think the problem is that this definition of "asynchronous" is very
> narrow. Yes, you don't need to wait for a request to finish before you
> issue a new one. But you'd still be bound to HTTP's request/response
> scheme in general.
>   

WebSockets uses HTTP so it is hardly immune to the request/response 
behaviour of its underlying protocol (including the stream nature of TCP).

Besides this statement appears to be based on the assumption that the 
server MUST wait for additional client requests to send each "message". 
However the specification allows the server to send "chunked" or 
"multipart" data in a variety of ways so full asynchronous communication 
is acheivable by making the response chunks  part of one long HTTP 
multipart response and allowing the javascript API to access the 
incoming data while the response is incomplete. It can be simplified for 
the end-user by allowing the raw server response to be read by bytes, 
lines or "parts". If you need more channels (ie, to pass messages while 
sending a large file) then you simply open more WebSockets and let the 
server handle multiplexing via cookies or message ids.

> However, web authors might want to employ other schemes as well, for
> example server-sided asynchronous notifications ("pushing"),
>   

Client opens a connection. handshakes, then leaves connection open 
listening for "pushed" parts in the response stream.

> client-sided notifications that don't need to be replied

HTTP has a status code specifically for this purpose.

>  or requests that can be answered out-of-order.

The order of data is controlled by the order it is sent by the 
application. There is no requirement for the requests and 
responses/parts to be synchronised. Especially if the server responses 
consist of a single multi-part "response".



I'm not advocating against WebSockets, just its current definition. In 
particular it tries to solve things that HTTP/1.1 already handles. I 
believe we should be thinking of WebSockets as a Javascript API, not a 
new communications protocol for the simple reason that HTTP is already a 
very suitable and widely deployed protocol. What authors (especially 
AJAX authors) are missing is a reliable way to use HTTP's existing 
asynchronous connection support.

Here are my issues with WebSockets as currently defined:

1.) Request must have a <scheme> component whose value is either "ws" or 
"wss"

The "scheme" should be HTTP(S). WebSockets should be the API.

2.) The message  event is fired when when data is received for a 
connection.

What "data"? A byte, a line, a chunk, the whole response? The spec isn't 
clear. I'd also recommend adding a connection.read( max_bytes ) method 
as used by Python and most languages to let the author receive bytes at 
a frequency appropriate to the application (eg, a game might want to 
frequently poll for small updates).

3.) If the resulting absolute URL has a <port> component, then let port 
be that component's value; otherwise, if secure is false, let port be 
81, otherwise let port be 815.

No, no, no! Don't let paranoia override common sense. Not all websocket 
applications will have the luxury to run on these ports (multiple web 
servers, shared host, tunnelled connections, 2 websocket apps on one 
host, etc...).

4.) The whole handshake is too complex.

There are many firewalls, proxies and servers that legimately insert, 
change, split, or remove HTTP headers or modify their order. This is 
also likely if the service being provided sits on top of a 
framework/server (such as Coldfusion/IIS). Also what happens if HTTP/1.2 
is sent? These will break the WebSocket handshake as currently defined.

rfc 2616 section 3 says: The version of an HTTP message is indicated by 
an HTTP-Version field in the first line of the message.
HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT

How many non-http servers send this? Probably none. I recommend the 
handshake simply read about 256 bytes of the server response and check 
that it contains a valid HTTP version field and one or more valid HTTP 
headers and optional end-of-headers marker. If ALL headers semantically 
validate  (ie, with the regex [A-Za-z-]: \s(.*?)\r\n) then it is 
reasonably safe to assume it is a real HTTP or WebSockets service. If 
the headers validate then the client repeats the process until the 
message body is reached. At this point we check our collected headers 
for at least one "Accept" header containing "WebSocket" (assuming that 
being a "WebSocket" rather than a basic pipelined HTTP connection is 
even useful).

5.) URI parsing specification

The current proposal spells out the URI/path parsing scheme. However 
this should be treated EXACTLY like HTTP so the need to define it in the 
spec is redundant. It is enough to say that the resource may be 
requested using a GET or POST request. Same with cookie handling, 
authorization and other HTTP headers. These should be handled by the 
webserver and/or application exactly as normal, there is no need to 
rewrite the rules simply because the information flow is asynchronous.

6.) Data framing specification

Redundant because HTTP already provides multiple methods of data segment 
encapsulation including "Content-Length", "Transfer-Encoding" and 
"Content-Type". Each of these have sub-types suitable for a range of 
possible WebSocket applications. Naturally it is not necessary for the 
client or server to support them all since there are HTTP headers 
explicitly designed for this kind of negotiation. The WebSocket should 
however define at least one fallback method that can be relied on (I 
recommend "Content-Length",  "Transfer-Encoding: chunked" and 
"Content-Type: multipart/form-data" as MUST requirements).

7.) WebSockets needs a low-level interface as well

By "dumbing down" the data transfer into fired events and wrapping the 
data segments internally the websocket hides the true communication 
behind an abstract object. This is a good thing for simplicity but 
extremely limiting for authors wanting to fine-tune an application or 
adapt to future protocols. I strongly recommend that rawwrite() and 
rawread() methods be made available to an OPEN (ie, 
authenticated/handshaked) websocket to allow direct handling of the 
stream. It would be understood that authors using these methods must 
understand the nature of both HTTP and websockets. In the same way a 
settimeout() method should be provided to control blocking/non-blocking 
behaviour. I can't stress enough how important these interfaces are, as 
they may one day be required to implement WebSockets 2.0 on "legacy" or 
broken HTML5 browsers.

8.) Origin: / WebSocket-Origin:

Specifying clients allowed to originate a connection is a disaster 
waiting to happen for the simple reason that sending your origin is a 
privacy violation in the same vain as the referrer field. Any 
open-source browser or privacy plugin will simply disable or spoof this 
since it would allow advertising networks to track people by ad-serving 
via websockets. Such tracking undermines the security of anonymising 
proxies (as the "origin" may be a private site or contain a client id). 
Using origin as a required field essentially makes the use of "referrer" 
mandatory. If a websocket wants to restrict access then it will have to 
use credentials or IP ranges like everything else.

9.) WebSocket-Location

The scenario this is supposed to solve (that an application makes a 
mistake about what host it's on and somehow sends the wrong data) is 
contrived. What's more likely to happen is that a server application has 
trouble actually knowing its (virtual) hostname (due to a proxy, 
mod_rewrite, URL masking or other legitimate redirect) and therefore NO 
clients can connect. It isn't uncommon for the host value passed to a 
CGI script and the hostname returned by the environment (ie, via uname 
or OS library) to conflict. Then there is the matter of an SSL 
connection (no host header available). I'm having trouble determining 
why this should even matter. I suspect most simple applications/wrappers 
will just echo back the host header sent by the client so if a mistake 
is made it's likely to go unnoticed anyway.

10.) To close the Web Socket connection, either the user agent or the 
server closes the TCP/IP connection. There is no closing handshake.

HTTP provides a reliable way of closing a connection so that all parties 
(client, server and proxies) know why the connection ended. There is no 
reason for websockets to not follow this protocol and close the 
connection properly.


In conclusion, the current specification of WebSockets re-invents 
several wheels and does so in ways that are overly complex, error-prone 
and yet seriously limited in functionality. The whole concept needs to 
be approached from the position of making HTTP's features (which are 
already implemented in most UAs) available to Javascript (while 
preventing the exploit of non-HTTP services). I do not believe this is 
difficult if my recommendations above are followed. I do not wish to be 
overly critical without contributing a solution, so if there are no 
serious objections to the points I've made I will put time into 
reframing my objections as a compete specification proposal.


Shannon


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.whatwg.org/pipermail/whatwg-whatwg.org/attachments/20080723/ac285de0/attachment.htm>
Received on Tuesday, 22 July 2008 23:17:55 UTC

This archive was generated by hypermail 2.4.0 : Wednesday, 22 January 2020 16:59:03 UTC