Socket Programming EE 122 Intro to Communication Networks Fall 2006 MW 4 5 30 in Donner 155 Sukun Kim Vern Paxson TAs Dilip Antony Joseph and Sukun Kim http inst eecs berkeley edu ee122 Materials with thanks to Artur Rivilis Jennifer Rexford Ion Stoica and colleagues at Princeton and UC Berkeley 1 Announcements Project 1 out today due Thu Sep 28 Vern Dilip away this week I will be covering Dilip s Friday section office hours in my office No Lecture Wed Sept 13 2 1 Overview Socket Programming how applications use the network Sockets are a C language programming interface between Layer 7 applications and Layer 4 transport Interface is quite general and fairly abstract Use of interface differs somewhat between clients servers 3 Socket End Point of Communication Sending message from one process to another Message must traverse the underlying network Process sends and receives through a socket In essence the doorway leading in out of the house Socket as an Application Programming Interface Supports the creation of network applications User process User process socket socket Operating System Operating System 4 2 Identifying the Receiving Process Sending process must identify the receiver Address of the receiving end host Plus identifier port that specifies the receiving process Receiving host Destination address uniquely identifies the host IP address is a 32 bit quantity Receiving process Host may be running many different processes Destination port uniquely identifies the socket Port number is a 16 bit quantity 5 Using Ports to Identify Services Server host 128 2 194 242 Client host Service request for 128 2 194 242 80 i e the Web server Web server port 80 OS Client Echo server port 7 Service request for 128 2 194 242 7 i e the echo server Client Web server port 80 OS Echo server port 7 6 3 Knowing What Port Number To Use Popular applications have well known ports E g port 80 for Web and port 25 for e mail Well known ports listed at http www iana org Or see etc services on Unix system Well known vs ephemeral ports Server has a well known port e g port 80 By convention between 0 and 1023 privileged Client gets an unused ephemeral i e temporary port By convention between 1024 and 65535 Uniquely identifying the traffic between the hosts The two IP addresses plus the two port numbers Sometimes called the four tuple And underlying transport protocol e g TCP or UDP With the above called the five tuple 7 Delivering the Data Division of Labor Network Deliver data packet to the destination host Based on the destination IP address Operating system Deliver data to the destination socket Based on the protocol and destination port Application Read data from the socket Interpret the data e g render a Web page 8 4 UNIX Socket API Socket interface Originally provided in Berkeley UNIX Later adopted by all popular operating systems Simplifies porting applications to different OSes In UNIX everything is like a file All input is like reading a file All output is like writing a file File is represented by an integer file descriptor System calls for sockets Client create connect write send read close Server create bind listen accept read recv write send close 9 Types of Sockets Different types of sockets implement different service models Stream SOCK STREAM Datagram SOCK DGRAM Stream socket TCP Connection oriented includes establishment termination Reliable lossless and in order delivery guaranteed At most once delivery no duplicates E g SSH HTTP Web SMTP email Datagram socket UDP Connectionless just data transfer of packets Best effort delivery possibly lower variance in delay E g IP Telephony Skype simple request reply protocols hostname address lookups via DNS time synchronization via NTP 10 5 Using Stream Sockets No need to packetize data Data arrives in the form of a byte stream Receiver needs to separate messages in stream TCP may send the messages joined together Hi there Hope you are well or may send them separately or might even split them like Hi there Ho and pe you are well application transport network data link physical User application sends messages Hi there and Hope you are well separately 11 Recovering message boundaries Stream socket data separation Use records data structures to partition data stream How do we implement variable length records size of record A fixed length record B C fixed length record 4 variable length record What if field containing record size gets corrupted Not possible Why 12 6 Datagram Sockets User packetizes data before sending Maximum size of 64 KB Using previous example Hi there and Hope you are well definitely sent in separate packets at network layer Message boundaries preserved But note your message had better fit within 64 KB or else you ll have to layer your own boundary mechanism on top of the datagram delivery anyway 13 Typical Client Program Prepare to communicate Create a socket Determine server address and port number Initiate the connection to the server Exchange data with the server Write data to the socket Read data from the socket Note single socket supports both reading and writing Do stuff with the data e g render a Web page Close the socket 14 7 Creating a Socket socket Operation to create a socket int socket int domain int type int protocol Returns a descriptor or handle for the socket Originally designed to support any protocol suite Domain protocol family Use PF INET for the Internet Type semantics of the communication SOCK STREAM reliable byte stream SOCK DGRAM message oriented service Protocol specific protocol UNSPEC unspecified No need for us to specify since PF INET plus SOCK STREAM already implies TCP or 15 SOCK DGRAM implies UDP Connecting the Socket to the Server Translate the server s name to an address struct hostent gethostbyname char name name the name of the host e g www cnn com Returns a structure that includes the host address Or NULL if host doesn t exist Identifying the service s port number struct servent getservbyname char name char proto E g getservbyname http tcp Establishing the connection int connect int sockfd struct sockaddr server address socketlen t addrlen Arguments socket descriptor server address and address size 16 Returns 0 on success and 1 if an error occurs 8 Sending and Receiving Data Sending data ssize t write int sockfd void buf size t len Arguments socket descriptor pointer to buffer of data to send and length of the buffer Returns the number of characters written and 1 on error Receiving data ssize t
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