I am confused at how SSE, despite using HTTP, is able to maintain the connection.
When I look at Web sockets, my understanding is that the most that HTTP is used is only for establishing the TCP connection and asking to upgrade to WS protocol. After that, the client and server both use the WS protocol which is why the connection can be persisted.
With SSE, there is no special protocol, it's just HTTP. Don't browsers have an HTTP TTL? How is the connection persisted beyond the TTL then?
Other HTTP polling mechanisms still suffer from timeout. SSE isn't a polling mechanism, but it still uses HTTP.
Related
I'm implementing HTTP over TLS proxy server (sni-proxy) that make two socket connection:
Client to ProxyServer
ProxyServer to TargetServer
and transfer data between Client and TargetServer(TargetServer detected using server_name extension in ClientHello)
The problem is that the client doesn't close the connection after the response has been received and the proxy server waits for data to transfer and uses resources when the request has been done.
What is the best practice for implementing this project?
The client behavior is perfectly normal - HTTP keep alive inside the TLS connection or maybe even a Websocket connection. Given that the proxy does transparent forwarding of the encrypted traffic it is not possible to look at the HTTP traffic in order to determine exactly when the connection can be closed. A good approach is therefore to keep the connection open as long as the resources allow this and on resource shortage close the connections which were idle (no traffic) the longest time.
I'm not clear why the handshake for WebSocket is HTTP. Wiki says "The handshake resembles HTTP so that servers can handle HTTP connections as well as WebSocket connections on the same port." What is the benefit of this? Once you start communicating over WebSocket you are using port 80 also...so why can't the initial handshake be in WebSocket format?
Also, how do you have both WebSocket and HTTP servers listening on port 80? Or is it typically the same application functioning as HTTP and WebSocket servers?
Thanks y'all :)
WebSockets are designed to work almost flawlessly with existing web infrastructures. That is the reason why WS connections starts as HTTP and then switches to a persistent binary connection.
This way the deployment is simplified. You don't need to modify your router's port forwarding and server listen ports... Also, because it starts as HTTP it can be load balanced in the same way that a normal HTTP request, firewalls are more lean to let the connection through, etc.. etc... Last but not the least, the HTTP handshake also carry cookies, which it is great to integrate with the rest of the app in the same way that AJAX does.
Both, traditional HTTP request-response and WS, can operate in the same port. Basiclally the WS client sends a HTTP request asking for "Upgrade:websocket", then if the server accepts the WS connections, replies with a HTTP response indicating "101 Switching Protocols", from that point the connection remains open and both ends consider it as a binary connection.
Websocket is designed in such a way that its servers can share a port with HTTP servers, by having its handshake be a valid HTTP Upgrade request.
I have a doubt in this design philosophy.
Any ways the WebSocket Protocol is an independent TCP-based protocol.
Why would we need this HTTP handshake(upgrade request) and a protocol switching. Instead why can't we directly(& independently) follow a websocket like protocol?
To quote from the IETF 6455 WebSocket spec:
The WebSocket Protocol attempts to address the goals of existing
bidirectional HTTP technologies in the context of the existing HTTP
infrastructure; as such, it is designed to work over HTTP ports 80
and 443 as well as to support HTTP proxies and intermediaries, even
if this implies some complexity specific to the current environment.
However, the design does not limit WebSocket to HTTP, and future
implementations could use a simpler handshake over a dedicated port
without reinventing the entire protocol.
In other words, there is a vast infrastructure for HTTP and HTTPS that already exists (proxies, firewalls, caches, and other intermediaries). In order to increase the chances of being adopted widely, the WebSocket protocol was designed to allow adjustments and extensions to the existing infrastructure without having to recreate everything from scratch to support a new protocol on a dedicate port.
It's also important to note that even if WebSocket protocol were to get rid of the HTTP compatible handshake, it would still need a handshake of almost equivalent complexity to support security requirements of the modern web so the browser and server can validate each other and to support CORS (cross-origin request sharing) securely. Even "raw" Flash sockets do a handshake with the server via the security policy request prior to creating the actual socket.
I've observed a HTTP 1.1 Server implementation, which terminates a client connection as soon as it detects a client-side connection shutdown of its outgoing channel (or rather, either before or after sending a proper http response). Is this a conforming HTTP 1.1 implementation?
RFC 2616 Section 8.1.4 seems to suggest this is to be the proper behaviour:
When a client or server wishes to time-out it SHOULD issue a graceful
close on the transport connection. Clients and servers SHOULD both
constantly watch for the other side of the transport close, and
respond to it as appropriate.
...
Servers SHOULD NOT close a connection in the middle of transmitting a response, unless a network or client failure is suspected.
Am I interpreting it right? Is there a more explicit reference about half-closed connection handling in the context of HTTP 1.1?
As far as i know, thats is all we need to know about Half-closed connections.
The server will only close the connection if it detects that the client closed it (it can ben when the server is about to write to the socket) or at the end of the request, if it does not support connection: keep-alive.
The client can disconnect any time, but it should tell the server why is it disconnecting (time_out, request cancel). But it is not very used by those who write sockets components. They just close the socket when they need to force a time_out.
But the client implementation is not the problem. You should worry about server implementation since suffer a lot with those unexpected disconnects.
EDIT
Maybe those links can help you.
Transmission Control Protocol - Functional Specification
TRANSMISSION CONTROL PROTOCOL
From this article on Wikipedia:
Keepalive messages were not officially
supported in HTTP 1.0. In HTTP 1.1 all
connections are considered persistent,
unless declared otherwise.
Does this mean that using this
mechanism I can actually simulate a
TCP socket connection?
Using this can I make a Server
"push" data to a client?
Are all HTTP connections, even the
one I am using to connect to Stack
Overflow "HTTP persistent"?
Does the COMET technology of
server push use this mechanism of
HTTP persistent connection to push
data to clients?
Does this mean that using this mechanism I can actually simulate a
TCP socket connection?
Not really, sockets have MANY more features and flexibility.
Using this can I make a Server "push" data to a client?
Not directly, it's still a request/response protocol; the persistent connection just means the client can use the same underlying socket to send multiple requests and receive the respective responses.
Are all HTTP connections, even the one I am using to connect to Stack
Overflow "HTTP persistent"?
Unless your browser (or a peculiar server) says otherwise, yes.
Does the COMET technology of server push use this mechanism of HTTP
persistent connection to push data to
clients?
Kinda (for streaming, at least), but with a lot of whipped cream on top. There are other Comet implementation approaches, such as hidden iframes and AJAX long polling, that may not require persistent connections (which give some firewalls &c the fits anyway;-).
Actually, the HTTP server can "push" data to a connected http client without the client requesting it. See "HTTP server push" at http://en.wikipedia.org/wiki/Push_technology. However it does seem to be commonly implemented.