2: Application Layer 1

Chapter 2Application Layer

Computer Networking: A Top Down Approach, 5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.

2: Application Layer 2

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 applications 2.7 Socket

programming with TCP 2.8 Socket

programming with UDP

2: Application Layer 3

recv and send

recv may be used instead of read (and is recommended):

n = recv(fd, buffer, count, flags);

The first three arguments are the same as for read. The flags argument provides additional flexibility. Interesting flags are MSG_DONTWAIT and MSG_PEEK. Refer to recv(2) for details.

2: Application Layer 4

recv and send

Similarly send may be used instead of write (and is also recommended):

n = send(fd, buffer, count, flags);

Interesting flags are MSG_DONTWAIT, MSG_MORE and MSG_DONTROUTE. Refer to send(2) for details.

2: Application Layer 5

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 applications 2.7 Socket

programming with TCP 2.8 Socket

programming with UDP

2: Application Layer 6

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 viewpoint

UDP provides unreliable transfer of groups of bytes (“datagrams”)

between client and server

2: Application Layer 7

Client/server socket interaction: UDP

Server (running on hostid)

closeclose()

read datagram usingrecvfrom

create socket,

clifd = socket()

Client

Create datagram with server IP andport=x; send datagram via sendto

create socket,port= x.srvfd = socket()bind()

read datagram usingrecvfrom

write reply usingsendtospecifying client address,port number

2: Application Layer 8

UDP Programming

The client functions for passing UDP datagrams are: socket(), sendto(), and recvfrom(). It is not necessary for the client to connect() with the server. (Although connect() can be used.)

The server uses socket(), bind(), recvfrom(), and sendto(). The server does not accept() a connection from the client, instead recvfrom() returns the data and client address.

2: Application Layer 9

UDP Programming

The sendto() routine takes a destination address structure as an argument:

typedef struct sockaddr SA;struct sockaddr_in srvaddr_in;memset(&srvaddr_in, 0, sizeof(srvaddr_in));srvaddr.sin_family = AF_INET;srvaddr.sin_port = htons(9000);inet_pton(AF_INET, “10.10.0.9”,

&srvaddr_in.sin_addr.s_addr);sfd = socket(AF_INET, SOCK_DGRAM, 0);sendto(sfd, msg, msglen, 0, (SA *)&srvaddr_in, sizeof(srvaddr_in));

2: Application Layer 10

UDP Programming

recvfrom() returns an address as an argument. The address is used by the server to respond to the proper client:

n = recvfrom(sockfd, mesg, mesg_len, 0, (SA *)&cliaddr_in, &len);sendto(sockfd, mesg, n, 0, (SA *)&cliaddr_in, len);

A NULL address may be used in recvfrom() if we do not care about the address.

Refer to udp_server.cpp for a complete example of UDP socket programming (a modified echo server).

In-class Exercise: Write a udp_client.cpp program to communicate with udp_server.cpp. It should prompt the user for input, send the input to the server, and then read and display the response.

2: Application Layer 11

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 P2P: BitTorrent, Skype

socket programming

2: Application Layer 12

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

Important themes: control vs. data msgs

in-band, out-of-band

centralized vs. decentralized

stateless vs. stateful reliable vs. unreliable

msg transfer “complexity at

network edge”