Networking Tutorial

Special Interest Group for Software EngineeringLuke Rajlich

What will be covered

● Basics of networking● Simple network client● Simple network server● Transform / Echo server

Basics of Networking

● What is a socket?– Input / Output port used for transmitting data over a

network– In Unix, a socket is actually a file descriptor! (called a

special file) It's one of those virtualization things● How do I refer or address a specific socket?

– A socket can be addressed by an IP address and port.– IP address is a way to find a computer on a network.– Port is a way by which the operating system directs

traffic to specific applications. (EX: Port 80 is used for HTTP requests.)

Types of Sockets

● Datagram Socket– No connection, simply build a packet and send it to the

destination.– No guarantee of completion, receipt, or message accuracy– Used in UDP protocol

● Stream Socket– A connection oriented socket. Connection is established– Messages are acknowledged when received. Acknowledge

bytes received to ensure the correctness of message– In actuality, built on top of datagram socket with extra

messages to maintain connection state– Used in TCP protocol

Basics of Networking, Cont.

● “Network Order”– You want to send data over a socket, you must put into

serial format, that is, the data must be a continuous chunk of bits.

– There is a specific order of bits as agreed upon on the internet is known as network order.

– Operating system may have different order of bits for certain data types....

– 'htonl', 'htons', 'ntohl', 'ntohs' functions ● Basic Communication Model

A Simple Socket Client

● Our goal: An application that opens a socket and downloads google's webpage

● Steps– Prerequisites to make it compile– Create a socket– Lookup www.google.com IP address via DNS– Create a struct containing the info to connect to Google– Connect to google's HTTP port (80)– Send HTTP GET request– Recv a set of bytes

Create A Socket

● int socket(int domain, int type, int protocol);– Domain:– Type: SOCK_STREAM or SOCK_DGRAM for

stream and datagram sockets, respectively– Protocol: use 0 to have socket automatically pick

correct one– Return: an integer called the socket file descriptor. aka

“sockfd” in future parts of tutorial.

Lookup Google via DNS

● struct hostent *gethostbyname(const char *name);– Lookup Call, returns a struct of type hostent containing

the information about the host looked up● hostent is a struct containing host entity

information.– Members

● h_name

Create a struct with Google's address

– sin_family: AF_INET (for our purposes)● googleaddr.sin_family = AF_INET;

– sin_port: port number in network order● googleaddr.sin_port = htonl(80);

– sin_addr: – sin_zero: must be set to all zero's

● memset(&(googleaddr.sin_zero), '\0', 8);

Connect to Google

● int connect(int sockfd, struct sockaddr *serv_addr, int addrlen);– Sockfd: Socket file decriptor– serv_addr: constructed in previous slide– Addrlen: length of the struct serv_addr

● sizeof(struct sockaddr)– Return: -1 if error.

Make HTTP GET Request

● Send a message using the connected socket

● int send(int sockfd, const void *msg, int len, int flags);– Sockfd: socket file descriptor to use to send message– Msg: a pointer to a stream of bytes of a message.

● Specific byte sequence for HTTP GET: ??– Len: the length of a message in number of bytes– Flags: 0. For our purposes.

Receive response from Google

● int recv (int sockfd, void *buf, int len, unsigned int flags);– sockfd: Socket file descriptor– Buf: pointer to a buffer – Len: maximum length of the buffer– Flags: 0. for our purposes.– Return: number of bytes read

● -1: error

Extra Special Considerations

● Serialization– You want to send data over a socket, you must put into

serial format, that is, the data must be a continuous chunk of bytes.

– Must thing about how to convert data structures into a serial format