4061 Session 25 (4/17)

Today

• Briefly: Select and Poll

• Layered Protocols and the Internets

• Intro to Network Programming

Today’s Objectives• Understand enough networking to get

going with sockets programming

• Compare Berkeley Sockets with other forms of IPC

• Write simple code for a network client

Race Conditions in Select

• Question from last time

• Answer: yes, there may be race conditions if multiple processes are interacting with the same I/O channels.

• Example

Network Programming

• Obviously, much of what we do these days involves different computers talking with one another– These computers may be physically

copresent or separated by oceans

System Calls for Networking

• Again, we are interested in understanding the system calls which the OS provides, such as calls to:– connect to another machine– send/receive data

Scope: Networking Info

• We will skip the technical details of how networking is implemented– Take 4211 (etc.) or read a networking book

for more info

• But there are a few details that we all should know

Abstraction Design Goals

• A machine may be connected to a network with a WIFI connection, or with an ethernet connection– We do not want to have to write code for each

case

• Similarly, there are several other features that we want to work automatically– Routing– Error Checking

Layered Protocols

• To get this abstraction, networks are organized as a stack of layers– Each layer provides a service to the layer above

(shielding that layer from implementation details)– Each layer is built on the layer below (using the

services provided)

• Layer n on one machine transacts with layer n on another machine– By way of a protocol: the rules and conventions used

in an abstraction layer

IP Layer

• Unique global addressing– Ensure that two computers on the Internet can

identify one another

• IPv4– 32 bit (4 byte) addresses (~4 billion)– E.g., the U of M’s Web server has the address

64.233.167.99

• Makes no guarantees about correctness or arrival (leaves that to upper layers)

Transport Layer

• Responsible for– end-to-end connection– error recovery– ensuring complete data transfer

• TCP– Connections, reliable, in-order delivery– Example apps: www, email, ssh

• UDP– Connectionless, unreliable, out-of-order, fast– Example apps: VoIP, online games

Routing

• A path between two computers is a series of hops– Connected by routers, computers whose job

is to figure out where your information should go in the next hop (to get it closer to its destination)

• The unit of information that’s routed is called a packet

• Different packets may take different routes

Ports

• An IP address identifies a machine, while a port identifies a communication endpoint

• One machine may serve both ssh and www…to allow this, sshd and httpd each bind themselves to a different port

• There are well-known ports (below #1024):– http: 80– https: 443– ftp: 21

• On linux, /etc/services

Internet Sockets

• An Internet Socket (or just “socket”) is an endpoint for communication– It is associated with an IP address and port– It is associated with a protocol on the

transport layer of TCP/IP (usually TCP or UDP)

Berkeley Sockets API

• An API for communications via ports• Designed to allow C programmers to

perform IPC across networks• Released in 1983• Still standard API, replicated on many

modern platforms and languages– Windows XP (winsock is based on BSD

sockets)– Python

Berkeley Sockets API

• Bi-directional (contrast with pipes) and full-duplex

• Can be used to perform IPC between processes on a single machine, or over a network

• telnet: open a socket from the command line