Chapter 2: Application layer

2: Application Layer 1

Chapter 2: Application layer 2.1 Principles of

network applications 2.2 Web and HTTP Internet gaming 2.3 FTP 2.4 Electronic Mail

SMTP, POP3, IMAP 2.5 DNS

2.6 P2P file sharing VOIP 2.8 Socket

programming with TCP 2.9 Socket

programming with UDP

2.10 Building a Web server


Definition also called IP Telephony, Internet telephony,

Broadband telephony, Broadband Phone and Voice over Broadband

the routing of voice conversations over the Internet or through any other IP-based network

Cisco IP Phone 7941G


Big Picture Modes of operation:

PC to PC PC to phone Phone to PC Phone to Phone

Traffic go through Packet Switched Network instead of Public Switched Telephone Network (PSTN)

From Wikipedia, the free encyclopedia


Challenges Quality of Service (QoS)

Internet provides best of service No guarantee for latency, jitter…

Need Internet connection Home broadband is not reliable

Power issue VOIP phone, Cable Modem/DSL, Computer Primary reason for not using VOIP for

emergency calls• Second reason is location identification is hard for

VOIP


Challenges Security

Most unencrypted VOIP spam challenges

Integration into global telephone number system

Emergency call availability & functionality Power, Internet connection Call routing, location service


QoS

Deal with Jitter Smoothed by playback buffer Will cause more delay in playback Too much delayed packets will be discard (dropped)

Bandwidth 64 kbps or less Depends on codec and use of silence suppression



network applications 2.2 Web and HTTP Internet gaming 2.3 FTP 2.4 Electronic Mail


2.6 P2P file sharing VOIP 2.7 Socket




Socket programming

Socket API introduced in BSD4.1 UNIX, 1981 explicitly created, used, released by apps client/server paradigm two types of transport service via socket API:

unreliable datagram (UDP) reliable, byte stream-oriented (TCP)

Goal: learn how to build client/server application that communicate using sockets


Socket-programming using TCPSocket: an interface between application process

and end-end-transport protocol (UCP or TCP)Why socket?: A Layer seen by application, OS

transparent

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

host orserver

internet


Socket programming with TCPClient must contact server server process must first

be running server must have created

socket (door) that accepts client’s contact

Client contacts server by: creating client-local TCP

socket specifying IP address, port

number of server process When client creates socket:

client TCP establishes connection to server TCP

When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk

with multiple clients source port numbers

used to distinguish clients (more in Chap 3)

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint


Many Versions of Socket APIs Unix socket (berkeley socket) Winsock MacTCP ….

We introduce Unix socket API here Can program under SUN OS, Linux, etc A good tutorial on socket programming:

• http://beej.us/guide/bgnet/


Socket Descriptor Data StructureDescriptor TableDescriptor Table

01234

Family: AF_INETFamily: AF_INETService: SOCK_STREAMService: SOCK_STREAMLocal IP: 111.22.3.4Local IP: 111.22.3.4Remote IP: 123.45.6.78Remote IP: 123.45.6.78Local Port: 2249Local Port: 2249Remote Port: 3726Remote Port: 3726


TCP Client/Server Socket Overview

socket()

bind()

listen()

accept()

send()

recv()

recv()

close()

socket()

TCP Client

connect()

send()

recv()

close()

connection establishment

data request

data reply

end-of-file notification

TCP Server

bind()


What is a Socket?

socket returns an integer (socket descriptor) sockfd < 0 indicates that an error occurred socket descriptors are similar to file descriptors

• FILE *fid; fid=fopen(“test.txt”, “rt”);

AF_INET: associates a socket with the Internet protocol family

SOCK_STREAM: selects the TCP protocol SOCK_DGRAM: selects the UDP protocol

int sockfd; /* socket descriptor */if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) }

perror(“socket”); exit(1);}


Socket Structure (Client)struct sockaddr_in { short int sin_family; // Address family unsigned short int sin_port; // Port number struct in_addr sin_addr; // Internet address unsigned char sin_zero[8]; // all zero};

AF_INET

// Internet address (Network Byte Order)// (a structure for historical reasons)

struct in_addr { unsigned long s_addr; // that's a 32-bit long, or 4 bytes

};

… … Big-Endian (Network Byte Order)101 103102

1A 2D 3C 4BIP: 1A.2D.3C.4B100


Bind (Client)

int sockfd;struct sockaddr_in local_addr;

local_addr.sin_family = AF_INET;local_addr.sin_port = 0; // random assign a portlocal_addr.sin_addr.s_addr = INADDR_ANY; // use my IP address memset(&(local_addr.sin_zero), '\0', 8); // zero the rest of the struct

sockfd = socket(AF_INET, SOCK_STREAM, 0); // create an empty socketbind(sockfd, (struct sockaddr *)&local_addr, sizeof(struct sockaddr));

Localhostinfo


Remote Host Structure

hostent *hp;hp = gethostbyname(“mail.cs.ucf.edu”);

“132.170.108.1”

struct hostent { char *h_name; /* official name */ char **h_aliases; /* alias list */ int h_addrtype; /* address type */ int h_length; /* address length */ char **h_addr_list; /* address list */ }; #define h_addr h_addr_list[0] /* backward compatibility */

Longwood.cs.ucf.edu

struct sockaddr_in remote_addr;remote_addr.sin_family = AF_INET; remote_addr.sin_port = htons(80); // short, network byte order (big-endian) remote_addr.sin_addr = *((struct in_addr *)hp->h_addr);memset(&(remote_addr.sin_zero), '\0', 8); // zero the rest

mail.cs.ucf.edu

Remotehostinfo


Connect(), send(), recv() by Client

connect(sockfd, (struct sockaddr *)&remote_addr, sizeof(struct sockaddr);

Struct sockaddr sockaddr_in

After connecting to the remote sever….

char sendStr[100], recvStr[100];….numByteSend=send(sockfd, sendStr, strlen(sendStr), 0);…recvNumByte = recv(sockfd, recvStr, MaxDataSize, 0);close(sockfd);

Blocking call

Remote hostinfo

Local hostsocket


Partial Send() and recv()Due to multiple packets in transmission

#include <sys/types.h> #include <sys/socket.h> int sendall(int sockfd, char *buf, int *len) {

int total = 0; // how many bytes we've sent int bytesleft = *len; // how many we have left to send int n; while(total < *len) {

n = send(sockfd, buf+total, bytesleft, 0); if (n == -1) { break; } total += n; bytesleft -= n;

} *len = total; // return number actually sent here return n==-1?-1:0; // return -1 on failure, 0 on success

}


Socket Programming in Server No need to connect() a remote host Need to listen() on specified port Accept() a connection request

Generate a new socket for one connection• Support multiple connections

int sockfd, new_fd; struct sockaddr_in local_addr, remote_addr;

// assign local_addr (remember to use hton() to assign server port!)

socket(…); // create empty socket descriptorbind(…); //fill in local address and assigned port to the

socket descriptorlisten(sockfd, backLog); // backLog is the max no. of connections in

queuenew_fd = accept(sockfd, (struct sockaddr *)&remote_addr,

&sizeof(struct sockaddr_in))

New socket discriptorFollowing commun. through this


Socket Programming in Server: fork() for multi-connection service

while(1) { // main accept() loop sin_size = sizeof(struct sockaddr_in); new_fd = accept(sockfd, (struct sockaddr *)&remote_addr, &sin_size);printf("server: got connection from %s\n", inet_ntoa(remote_addr.sin_addr)); if (!fork()) { // this is the child process (fork() returns 0 in child process)

close(sockfd); // child doesn't need the listener send(new_fd, "Hello, world!\n", 14, 0); ………close(new_fd); exit(0);

} close(new_fd); // parent doesn't need this ………….

}

child

parent

parent


Fork() Tuotrial on fork():

http://www.erlenstar.demon.co.uk/unix/faq_2.html System call fork() is used to create child

process. It returns a process ID. After a new child process is created, both processes will execute the next instruction following the fork() system call.

On success: PID of the child process is returned in the parent's

thread of execution 0 is returned in the child's thread of execution

http://www.erlenstar.demon.co.uk/unix/faq_2.html



network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail


2.6 P2P file sharing 2.7 VOIP 2.8 Socket



2.10 Building a Web server


Socket programming with UDP

UDP: no “connection” between client and server

no handshaking sender explicitly attaches

IP address and port of destination to each packet

server must extract IP address, port of sender from received packet

UDP: transmitted data may be received out of order, or lost

application viewpointUDP provides unreliable transfer

of groups of bytes (“datagrams”) between client and server


UDP Socket Programming sockfd = socket(AF_INET, SOCK_DGRAM, 0)

No connect(), accept() Send() sendto(), recv() recvfrom()

Sendto() includes target address/port

SOCK_STREAM (tcp)


Chapter 2: Summary

Application architectures client-server P2P hybrid

application service requirements: reliability, bandwidth, delay

Internet transport service model connection-oriented, reliable:

TCP unreliable, datagrams: UDP

Our study of network apps now complete! specific protocols:

HTTP FTP SMTP, POP, IMAP DNS

Some applications Web Email DNS Internet gaming, VOIP P2P

socket programming


Chapter 2: Summary

typical request/reply message exchange: client requests info or

service server responds with

data, status code message formats:

headers: fields giving info about data

data: info being communicated

Most importantly: learned about protocols

control vs. data msgs in-band, out-of-band

(ftp) centralized vs.

decentralized stateless vs. stateful reliable vs. unreliable msg

transfer “complexity at network

edge”

Documents

Chapter 2: Application layer