Bidirectional Networking

[Bryan Olson]

Emanuele D'Arrigo wrote:
> Bryan Olson wrote:
>> Software firewalls will often simply refuse incoming connections. The
>> basic protection of the garden-variety home router comes from "network
>> address translation" (NAT), in which case TCP connections initiated from
>> the inside will generally work, regardless of port, and incoming
>> connections will fail.
> 
> Ok, I think I'm getting the picture here. So this means that in most
> circumstances where the data flow from the server is frequent the
> client initiates the connection, usually requests some initial data
> and keeps polling the server periodically, issuing new requests. In
> this context can the client simply keep the connection alive and
> listen for new data from the server coming at any time rather than
> actively issuing requests? 

Sure. You might include a method for the client to ensure the connection 
is alive, either with TCP keep-alive or your own messages over the 
connection.

> Are there drawbacks to this strategy? I.e.
> is there a limit to the number of simultaneous connections a server
> can keep alive? I've noticed that the socket pages mention a 5
> connections limit. Is that it? What if I want to make a virtual room
> with 20 people connected simultaneously?

Five is not the connection limit. Twenty connections is no problem, and 
modern systems should support hundreds by default, and thousands with a 
bit of care and tuning.

>>> or would it be the responsibility of the user
>>> to configure the firewall so that the application can receive a
>>> connection?
>> That can be a huge hassle. The first choice is for the application to
>> conform to popular firewall policies, so no special configuration is
>> required.
> 
> I agree, I'd rather have the user do nothing in this regard. I'm just
> wondering how it's done with data intensive application where the
> server needs to send new data to the client frequently. Does the
> client just keep the connection live at all time for the server to
> send stuff or does the client continuously open, sends a request,
> receives data and closes the connection every time? Here I'm thinking
> about an online game, with 100 players moving their avatars. Does the
> client requests their position nearly every frame?

I'd say keeping an open connection to each client is reasonable there. 
SocketServer and its descendants may not be sophisticated enough for 
such an application.


-- 
--Bryan