Network Fundamentals

20 Apr 2019

The Problem

* Communication between computers

* It's just sending/receiving bits

Two Min Issues

* Addressing

    * Specifying a remote computer and service

* Data transport

    * Moving bits back and forth

Network Addressing

* Machines have a hostname and IP address

* Programs/services have port number

Standard Ports

* Ports for common services are Pressigned

     21  FTP

     22  SSH

     23  Telnet

     25  SMTP  (Mail)

     80  HTTP  (Web)

    110  POP3  (Mail)

    119  NNTP  (News)

    443  HTTPs (web)

* Other port numbers may just be randomly assigned to programs by the operation system

Using netstart

* Use 'netstart' to view active network connections

    sheel: newstart -a

* Note: Must execute from the command shell on both Unix and Windows

Connections

* Each endpoint of network connection is always represented by a host and post #

* In python you write it out as a tuple (host, post)

Client/Server concept

* Each endpoint is a running program

* servers wait for incoming connections and provide a service (e.g: web, mail)

* Clients make connections to servers

Request/Response Cycle

* Most nerwork programs use a request/response model based on messages

* Client sends a request message (eg: HTTP)
        GET /index.html HTTP/1.0

* server sends back a response message
    HTTP/1.0 200 ok
    content-type: text/html
    content-length: 48823

    <html>
    .....

* The exact fromat depends on the appllication

Using Telnet

* As a debugging aid, telnet can be used to directly communicate with manu services
    telnet hostnmae portnum

Data Transport

* There are two basic types of communication

* Streams (TCP): computers establish a connection with each other and read/Write data in a continuous stream of byetes like a flie. This is the most common

* Datagrams (UDP): Computers send discrete packets (or messages) to each other. Each packet contains a collection of bytes, but each packet is separate and self-contained.

Sockets

* Programming abstraction for network code

* Socket: A communication endpoint

* Supported by socket library module

* Allows connections to be made and data to be transmitted in either direction

Socket Basics

* To create a socket
    import socket
    s = socket.socket(addr_family, type)

* Address families
    socket.AF_INET  internet protocol (IPv4)
    socket.AF_INET6 internet protocol (IPV6)

* Socket types
    socket.SOCK_STREAM connection baseed stream (TCP)
    socket.SOCK_DGRAM  Datagrams (UDP)

socket Types

* Almost all code will use one of following

    from socket import *

    s = socket(AF_INET, SOCK_STREAM)
    s = scoket(AF_INET, SOCK_DGRAM)

* Most common case: TCP connection
    s = socket(AF_INET, SOCK_STREAM)

Using a Scoket

* creating a socket is onlu th first step

    s = socket(AF_INET, SOCK_STREAM)

* Further use depends on application

* Server
    * Listen for incoming connections

* Client

    * Make an outgoing connection

TCP Client

* How to make an outgoing connection

    from socket import *
    s = socket(AF_INET, SOCK_STREAM)
    s.connect(('www.python.org', 80))
    s.send("GET /index.html HTTP/1.0\n\n")
    data = s.recv(10000)
    s.close()

* s.connect(addr) makes a connection
    s.connect(('www.python.org',80))

* Once connected, usd send(),recv() to transmit and receive data

* close() shuts down the connection