Chapter 2: Application layer
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Transcript of Chapter 2: Application layer
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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
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2: Application Layer 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
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2: Application Layer 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
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2: Application Layer 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
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2: Application Layer 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
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2: Application Layer 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
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2: Application Layer 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
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2: Application Layer 8
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
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2: Application Layer 9
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
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2: Application Layer 10
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
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2: Application Layer 11
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/
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2: Application Layer 12
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
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2: Application Layer 13
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()
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2: Application Layer 14
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);}
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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
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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 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
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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
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2: Application Layer 18
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
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2: Application Layer 19
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
}
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2: Application Layer 20
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
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2: Application Layer 21
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
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2: Application Layer 22
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
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2: Application Layer 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
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2: Application Layer 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
application viewpointUDP provides unreliable transfer
of groups of bytes (“datagrams”) between client and server
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2: Application Layer 25
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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2: Application Layer 26
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
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2: Application Layer 27
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”