3rd Edition: Chapter 2
Application Layer
2-*
Chapter 2
Application Layer
Computer Networking: A Top Down Approach
7th edition
Jim Kurose, Keith Ross
Pearson/Addison Wesley
April 2016
Application Layer
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Application Layer
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Chapter 2: outline
2.1 principles of network applications
2.2 Web and HTTP
2.3 electronic mail
SMTP, POP3, IMAP
2.4 DNS
2.5 P2P applications
2.6 Video streaming and content distribution networks
Application Layer
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Application Layer
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Chapter 2: application layer
our goals:
conceptual, implementation aspects of network application protocols
transport-layer service models
client-server paradigm
peer-to-peer paradigm
content distribution networks
learn about protocols by examining popular application-level protocols
HTTP
SMTP / POP3 / IMAP
DNS
creating network applications
socket API
Application Layer
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Some network apps
e-mail
web
text messaging
remote login
P2P file sharing
multi-user network games
streaming stored video (YouTube, Hulu, Netflix)
voice over IP (e.g., Skype)
real-time video conferencing
social networking
search
…
…
Application Layer
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Creating a network app
write programs that:
run on (different) end systems
communicate over network
e.g., web server software communicates with browser software
no need to write software for network-core devices
network-core devices do not run user applications
applications on end systems allows for rapid app development, propagation
application
transport
network
data link
physical
application
transport
network
data link
physical
application
transport
network
data link
physical
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Application architectures
possible structure of applications:
client-server
peer-to-peer (P2P)
hybrid of client-server and P2P
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Client-server architecture
server:
always-on host
permanent IP address
data centers for scaling
clients:
communicate with server
may be intermittently connected
may have dynamic IP addresses
do not communicate directly with each other
client/server
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P2P architecture
no always-on server
arbitrary end systems directly communicate
peers request service from other peers, provide service in return to other peers
self scalability – new peers bring new service capacity, as well as new service demands
peers are intermittently connected and change IP addresses
complex management
peer-peer
Application Layer
Advantages: self-scalability, cost-effectiveness
Challenges: security, performance, reliability, incentives, ISPs
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Hybrid of client-server and P2P
Skype
voice-over-IP P2P application
centralized server: finding address of remote party
client-client connection: direct (not through server)
Instant messaging
chatting between two users is P2P
centralized service: client presence detection/location
user registers its IP address with central server when it comes online
user contacts central server to find IP addresses of buddies
Application 2-*
Application Layer
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Processes communicating
process: program running within a host
within same host, two processes communicate using inter-process communication (defined by OS)
processes in different hosts communicate by exchanging messages
client process: process that initiates communication
server process: process that waits to be contacted
aside: applications with P2P architectures have client processes & server processes
clients, servers
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Sockets
process sends/receives messages to/from its socket
socket analogous to door
sending process shoves message out door
sending process relies on transport infrastructure on other side of door to deliver message to socket at receiving process
Internet
controlled
by OS
controlled by
app developer
transport
application
physical
link
network
process
transport
application
physical
link
network
process
socket
API: (1) choice of transport protocol; (2) ability to fix a few parameters (more on this later)
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Addressing processes
to receive messages, process must have identifier
host device has unique 32-bit IP address
Q: does IP address of host on which process runs suffice for identifying the process?
identifier includes both IP address and port numbers associated with process on host.
example port numbers:
HTTP server: 80
mail server: 25
to send HTTP message to gaia.cs.umass.edu web server:
IP address: 128.119.245.12
port number: 80
more shortly…
A: no, many processes can be running on same host
Application Layer
Port number: 16-bit unsigned integer
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Application Layer
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App-layer protocol defines
types of messages exchanged,
e.g., request, response
message syntax:
what fields in messages & how fields are delineated
message semantics
meaning of information in fields
rules for when and how processes send & respond to messages
open protocols:
defined in RFCs
allows for interoperability
e.g., HTTP, SMTP
proprietary protocols:
e.g., Skype
Application Layer
Application vs app layer protocol
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Application Layer
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What transport service does an app need?
data loss
some apps (e.g., file transfer, web transactions) require 100% reliable data transfer
other apps (e.g., audio) can tolerate some loss
timing
some apps (e.g., Internet telephony, interactive games) require low delay to be “effective”
throughput
some apps (e.g., multimedia) require minimum amount of throughput to be “effective”
other apps (“elastic apps”) make use of whatever throughput they get
security
encryption, data integrity, …
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Transport service requirements: common apps
application
file transfer
e-mail
Web documents
real-time audio/video
stored audio/video
interactive games
text messaging
data loss
no loss
no loss
no loss
loss-tolerant
loss-tolerant
loss-tolerant
no loss
throughput
elastic
elastic
elastic
audio: 5kbps-1Mbps
video:10kbps-5Mbps
same as above
few kbps up
elastic
time sensitive
no
no
no
yes, 100’s ms
yes, few secs
yes, 100’s ms
yes and no
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Internet transport protocols services
TCP service:
connection-oriented: setup required between client and server processes
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when network overloaded
does not provide: timing, minimum throughput guarantee, security
UDP service:
unreliable data transfer between sending and receiving process
does not provide: reliability, flow control, congestion control, timing, throughput guarantee, security, or connection setup,
Q: why bother? Why is there a UDP?
Application Layer
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Internet apps: application, transport protocols
application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
Internet telephony
application
layer protocol
SMTP [RFC 2821]
Telnet [RFC 854]
HTTP [RFC 2616]
FTP [RFC 959]
HTTP (e.g., YouTube),
RTP [RFC 1889]
SIP, RTP, proprietary
(e.g., Skype)
underlying
transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
TCP or UDP
Application Layer
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Securing TCP
TCP & UDP
no encryption
cleartext passwds sent into socket traverse Internet in cleartext
SSL
provides encrypted TCP connection
data integrity
end-point authentication
SSL is at app layer
apps use SSL libraries, that “talk” to TCP
SSL socket API
cleartext passwords sent into socket traverse Internet encrypted
see Chapter 8
Application Layer
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Application Layer
Socket programming
Socket API
introduced in BSD4.1 UNIX, 1981
A socket is explicitly created, used, released by apps
two types of transport service via socket API:
unreliable datagram
reliable, byte stream-oriented
a host-local,
application-created,
OS-controlled interface (a “door”) into which an
application process can both send and
receive messages to/from another application process
Goal: learn how to build client/server application that communicate using sockets
Application 2-*
socket
Socket
Socket Family
PF_INET denotes the Internet family
PF_UNIX denotes communication on the same host
PF_PACKET denotes direct access to the network interface (i.e., it bypasses the TCP/IP protocol stack)
Socket Type
SOCK_STREAM is used to denote a byte stream
SOCK_DGRAM is an alternative that denotes a message oriented service, such as that provided by UDP
The University of Adelaide, School of Computer Science
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Chapter 2 — Instructions: Language of the Computer
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PF_PACKET, SOCK_RAW equivalent to PF_INET, SOCK_PACKET but the 2nd one is obsolete
fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL)); or ETH_P_IP, ETH_P_IPV6, ETH_P_ARP, …
PF vs AF: Stevens and beej always use AF
PF_UNIX: communication on the same machine. TYPE = SOCK_STREAM, SOCK_DGRAM, SOCK_SEQPACKET (message oriented that preserves the order)
Socket-programming using TCP
Socket: a door between application process and end-end-transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one process to another
controlled by
application
developer
controlled by
operating
system
client or
server
controlled by
application
developer
controlled by
operating
system
client or
server
internet
Application 2-*
TCP with
buffers,
variables
socket
process
TCP with
buffers,
variables
socket
process
Socket programming with TCP
Client must contact server
server process must first be running
server must have created socket (door) that welcomes 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 2-*
application viewpoint
Client
process
client TCP socket
Stream jargon
stream is a sequence of bytes that flow into or out of a process.
input stream is attached to some input source for the process, e.g., keyboard or socket.
output stream is attached to an output source, e.g., monitor or socket.
Application 2-*
Creating a Socket
int sockfd = socket(socket_family, type, protocol);
The socket number returned is the socket descriptor for the newly created socket
int sockfd = socket (PF_INET, SOCK_STREAM, 0);
int sockfd = socket (PF_INET, SOCK_DGRAM, 0);
The combination of PF_INET and SOCK_STREAM implies TCP
Application 2-*
The University of Adelaide, School of Computer Science
*
Chapter 2 — Instructions: Language of the Computer
*
PF vs. AF: Stevens, beej always use AF
Client-Server Model with TCP
Server
Passive open
Prepares to accept connection, does not actually establish a connection
Server invokes
int bind (int socket, struct sockaddr *address, int addr_len)
int listen (int socket, int backlog)
int accept (int socket, struct sockaddr *address, int *addr_len)
Application 2-*
The University of Adelaide, School of Computer Science
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Chapter 2 — Instructions: Language of the Computer
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Client-Server Model with TCP
Bind
Binds the newly created socket to the specified address i.e. the network address of the local participant (the server)
Address is a data structure which combines IP and port
Listen
Defines how many connections can be pending on the specified socket
Application 2-*
The University of Adelaide, School of Computer Science
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Chapter 2 — Instructions: Language of the Computer
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Client-Server Model with TCP
Accept
Carries out the passive open
Blocking operation
Does not return until a remote participant has established a connection
When it does, it returns a new socket that corresponds to the new established connection and the address argument contains the remote participant’s address
Application 2-*
The University of Adelaide, School of Computer Science
*
Chapter 2 — Instructions: Language of the Computer
*
Client-Server Model with TCP
Client
Application performs active open
It says who it wants to communicate with
Client invokes
int connect (int socket, struct sockaddr *address, int addr_len)
Connect
Does not return until TCP has successfully established a connection at which application is free to begin sending data
Address contains remote machine’s address
Application 2-*
The University of Adelaide, School of Computer Science
*
Chapter 2 — Instructions: Language of the Computer
*
Client-Server Model with TCP
In practice
The client usually specifies only remote participant’s address and let’s the system fill in the local information
Whereas a server usually listens for messages on a well-known port
A client does not care which port it uses for itself, the OS simply selects an unused one
Application 2-*
The University of Adelaide, School of Computer Science
*
Chapter 2 — Instructions: Language of the Computer
*
Client-Server Model with TCP
Once a connection is established, the application process invokes two operations
int send (int socket, char *msg, int msg_len,
int flags)
int recv (int socket, char *buff, int buff_len,
int flags)
Application 2-*
The University of Adelaide, School of Computer Science
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Chapter 2 — Instructions: Language of the Computer
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Return #bytes written/read
TCP Client/Server Socket Interaction
Application 2-*
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