Networking

Connection between 2 computers to exchange messages.

Consists of

Physical connections: e.g. Ethernet wires, switches, Internet routers
Software standards - "Internet Protocol"

Applications

Some uses of networking, in relation to computer graphics, are:

Multi-player games
Shared VR environments
Discussion with shared whiteboard
Remote scientific instruments
Steering supercomputer simulations
3D web browsers

Networking Multiple Hosts

Communication between more than 2 hosts normally involves many pair-wise connections.

Peer-to-peer model

Networking Multiple Hosts

Server-based model

Server can be a simple reflector, a database, or a whole simulation in itself

Sockets

An Internet Protocol (IP) network connection uses sockets.

A socket connection involves:

an IP address (e.g. 128.205.4.175)
a port number (e.g. 80)
a protocol (e.g. TCP/IP)

Programming libraries can look up the IP address from a host name (e.g. www.buffalo.edu).

The port number and protocol are normally decided on in advance for a particular application.

(It is also possible to use dynamically generated port numbers.)

Network Data

A network connection allows one computer to send bytes of data to, or receive bytes of data from, another computer.

Everything beyond the exchange of raw data is higher-level interpretation of the data, handled by the application or a library.

Note that different computers can interpret a stream of bytes differently, depending on things such as "native byte ordering".

Functions to convert to & from "network byte order" help avoid problems.

Sending everything as text strings is an easy alternative.

UDP vs TCP

The two major protocols are TCP/IP and UDP/IP

TCP/IP:

Reliable
Stream-oriented
More overhead
Connection based

UDP/IP:

Low overhead
Packet-oriented
Not guaranteed
No explicit connection

For applications like real-time graphics, UDP is often preferred for its speed and simplicity. But a mix of TCP and UDP can also be useful.

Python Sockets

Sockets are implemented in the socket module.

Main functions for UDP:

`s = socket(AF_INET, SOCK_DGRAM)`	Create a UDP socket
`s.bind(('',portnum))`	Listen to a specific port
`s.sendto(data, (host, portnum))`	Send data to remote host
`data, addr = s.recvfrom(maxPacketSize)`	Receive data; return value is tuple of data and sender
`s.setblocking(0)`	Make the socket non-blocking. Trying to receive when no data is available will generate an exception.

Examples

Web-based Networking

Web tools such as PHP, Shockwave, and Python CGI scripts can do socket-based networking.

This can be used to communicate from a web page to a graphical program.

Example: send.php

Web-based Networking

Python's urllib or urllib2 can be used to fetch web data.

e.g. textures or sounds

    try:
        webdata = urllib2.urlopen(url).read()
    except:
        return None
    webdataAsFile = StringIO.StringIO(webdata)
    tex = Texture2D(webdataAsFile)

Example: texURL.py

Assignment 5

DUE: Working draft - Thursday, 7 December 2006, in class (2:00pm)
Final version - by Friday, 15 December 2006 (via e-mail, to depape@buffalo.edu)

Create an interactive, networked, graphical environment that can involve multiple participants.
Note that this is fairly vague; some options are:

2D or 3D
OpenGL or Panda3D
the networking can be like a multiplayer game (2 or more players with the same program, communicating via UDP or TCP), web-oriented (e.g. getting data from a server, which others can affect via a web interface), or anything else you come up with that involves remote connections and multple people.

You will have to demonstrate a functional version of your project in class on Dec 7. You may continue to work on it after that, and turn in the final version be Dec 15.
E-mail the final program, and any required data files, to me at depape@buffalo.edu

This document is by Dave Pape, and is released under a Creative Commons BY-2.0 License.