Chapter 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

2
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
3
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
4
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

5
Challenges

• Security
• Most unencrypted
• VOIP spam challenges
• Integration into global telephone number system
• Emergency call availability functionality
• Power, Internet connection
• Call routing, location service

6
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

7
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
• 2.7 Socket programming with TCP
• 2.8 Socket programming with UDP

8
Socket programming
Goal learn how to build client/server
application that communicate using sockets

• 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)

9
Socket-programming using TCP

• Socket an interface between application process
  and end-end-transport protocol (UCP or TCP)
• Why socket? A Layer seen by application, OS
  transparent

controlled by application developer
controlled by operating system
internet
host or server
host or server
10
Socket programming with TCP

• Client must contact server
• server process must first be running
• server must have created socket (door) that
  accepts clients 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)

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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/

12
Socket Descriptor Data Structure
Descriptor Table
Family AF_INET Service SOCK_STREAM Local IP
111.22.3.4 Remote IP 123.45.6.78 Local Port
2249 Remote Port 3726
0
1
2
3
4
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TCP Client/Server Socket Overview
TCP Server
socket()
TCP Client
bind()
socket()
listen()
bind()
accept()
connection establishment
connect()
data request
recv()
send()
data reply
send()
recv()
recv()
end-of-file notification
close()
close()
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What is a Socket?
int sockfd / socket descriptor / if
((sockfd socket(AF_INET, SOCK_STREAM, 0)) lt 0)
perror(socket) exit(1)

• socket returns an integer (socket descriptor)
• sockfd lt 0 indicates that an error occurred
• socket descriptors are similar to file
  descriptors
• FILE fid fidfopen(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

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Socket Structure (Client)
AF_INET
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_zero8
// all zero
// 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
IP 1A.2D.3C.4B
101
103
100
102


4B
Big-Endian (Network Byte Order)
1A
3C
2D
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Bind (Client)
int sockfd struct sockaddr_in local_addr local_
addr.sin_family AF_INET local_addr.sin_port
0 // random assign a port local_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 socket bind(sockfd, (struct sockaddr
)local_addr, sizeof(struct sockaddr))
Local host info
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Remote Host Structure
Longwood.cs.ucf.edu
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_list0 / backward compatibility
/
mail.cs.ucf.edu
132.170.108.1
hostent hp hp gethostbyname(mail.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-gth_addr) memset((remote_addr.sin_ze
ro), '\0', 8) // zero the rest
Remote host info
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Connect(), send(), recv() by Client
connect(sockfd, (struct sockaddr )remote_addr,
sizeof(struct sockaddr)
Remote host info
Local host socket
Struct sockaddr ? sockaddr_in
After connecting to the remote sever.
Blocking call
char sendStr100, recvStr100 . send(sockfd,
sendStr, strlen(sendStr), 0) recvNumByte
recv(sockfd, recvStr, MaxDataSize,
0) close(sockfd)
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Partial Send() and recv()

• Due to multiple packets in transmission
• include ltsys/types.hgt
• include ltsys/socket.hgt
• 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 lt len)
• n send(sockfd, buftotal, bytesleft, 0)
• if (n -1) break
• total n bytesleft - n
• len total // return number actually sent
  here
• return n-1?-10 // return -1 on failure, 0 on
  success

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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
socket() bind() listen(sockfd, backLog)
// backLog is the max no. of connections in
queue new_fd accept(sockfd, (struct sockaddr
)remote_addr, sizeof(struct sockaddr_in))
New socket discriptor Following commun. through
this
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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 .
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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

23
Chapter 2 Application layer

• 2.1 Principles of network applications
• 2.2 Web and HTTP
• 2.3 FTP
• 2.4 Electronic Mail
• SMTP, POP3, IMAP
• 2.5 DNS

• 2.6 P2P file sharing
• 2.7 VOIP
• 2.8 Socket programming with TCP
• 2.9 Socket programming with UDP
• 2.10 Building a Web server

24
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

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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)
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Chapter 2 Summary

• 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

• Application architectures
• client-server
• P2P
• hybrid
• application service requirements
• reliability, bandwidth, delay
• Internet transport service model
• connection-oriented, reliable TCP
• unreliable, datagrams UDP

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Chapter 2 Summary

• Most importantly learned about protocols

• 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

• 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