Chapter 2: Application layer
 2.1 Principles of
network applications


app architectures
app requirements
 2.2 Web and HTTP
 2.4 Electronic Mail
 SMTP, POP3, IMAP
 2.6 P2P applications
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
 2.5 DNS
2: Application Layer
1
Pure P2P architecture
 no always-on server
 arbitrary end systems
directly communicate peer-peer
 peers are intermittently
connected and change IP
addresses
 Three topics:
 File distribution
 Searching for information
 Case Study: Skype
2: Application Layer
2
File Distribution: Server-Client vs P2P
Question : How much time to distribute file
from one server to N peers?
us: server upload
bandwidth
Server
us
File, size F
dN
uN
u1
d1
u2
ui: peer i upload
bandwidth
d2
di: peer i download
bandwidth
Network (with
abundant bandwidth)
2: Application Layer
3
File distribution time: server-client
 server sequentially
sends N copies:

NF/us time
 client i takes F/di
time to download
Server
F
us
dN
u1 d1 u2
d2
Network (with
abundant bandwidth)
uN
Time to distribute F
to N clients using = dcs = max { NF/us, F/min(di) }
i
client/server approach
increases linearly in N
(for large N) 2: Application Layer
4
File distribution time: P2P
 server must send one
Server
F
u1 d1 u2
d2
copy: F/us time
us
 client i takes F/di time
Network (with
dN
to download
abundant bandwidth)
uN
 NF bits must be
downloaded (aggregate)
 fastest possible upload rate: us + Sui
dP2P = max { F/us, F/min(di) , NF/(us + Sui) }
i
2: Application Layer
5
Server-client vs. P2P: example
Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
Minimum Distribution Time
3.5
P2P
Client-Server
3
2.5
2
1.5
1
0.5
0
0
5
10
15
20
25
30
35
N
2: Application Layer
6
File distribution: BitTorrent
 P2P file distribution
tracker: tracks peers
participating in torrent
torrent: group of
peers exchanging
chunks of a file
obtain list
of peers
trading
chunks
peer
2: Application Layer
7
BitTorrent (1)
 file divided into 256KB chunks.
 peer joining torrent:
has no chunks, but will accumulate them over time
 registers with tracker to get list of peers,
connects to subset of peers (“neighbors”)
 while downloading, peer uploads chunks to other
peers.
 peers may come and go
 once peer has entire file, it may (selfishly) leave or
(altruistically) remain

2: Application Layer
8
BitTorrent (2)
Pulling Chunks
 at any given time,
different peers have
different subsets of
file chunks
 periodically, a peer
(Alice) asks each
neighbor for list of
chunks that they have.
 Alice sends requests
for her missing chunks
 rarest first
Sending Chunks: tit-for-tat
 Alice sends chunks to four
neighbors currently
sending her chunks at the
highest rate
 re-evaluate top 4 every
10 secs
 every 30 secs: randomly
select another peer,
starts sending chunks
 newly chosen peer may
join top 4
 “optimistically unchoke”
2: Application Layer
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BitTorrent: Tit-for-tat
(1) Alice “optimistically unchokes” Bob
(2) Alice becomes one of Bob’s top-four providers; Bob reciprocates
(3) Bob becomes one of Alice’s top-four providers
With higher upload rate,
can find better trading
partners & get file faster!
2: Application Layer
10
P2P: searching for information
Index in P2P system: maps information to peer location
(location = IP address & port number)
.
Instant messaging
File sharing (eg e-mule)
 Index maps user
 Index dynamically
names to locations.
tracks the locations of
files that peers share.
 When user starts IM
application, it needs to
 Peers need to tell
inform index of its
index what they have.
location
 Peers search index to
 Peers search index to
determine where files
determine IP address
can be found
of user.
2: Application Layer
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P2P: centralized index
original “Napster” design
1) when peer connects, it
informs central server:


Bob
centralized
directory server
1
peers
IP address
content
2) Alice queries for “Hey
Jude”
3) Alice requests file from
Bob
1
3
1
2
1
Alice
2: Application Layer
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P2P: problems with centralized directory
 single point of failure
 performance bottleneck
 copyright infringement:
“target” of lawsuit is
obvious
file transfer is
decentralized, but
locating content is
highly centralized
2: Application Layer
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Query flooding
 fully distributed
 no central server
 used by Gnutella
 Each peer indexes the
files it makes available
for sharing (and no
other files)
overlay network: graph
 edge between peer X
and Y if there’s a TCP
connection
 all active peers and
edges form overlay net
 edge: virtual (not
physical) link
 given peer typically
connected with < 10
overlay neighbors
2: Application Layer
14
Query flooding
 Query message
sent over existing TCP
connections
 peers forward
Query message
 QueryHit
sent over
reverse
Query
path
File transfer:
HTTP
Query
QueryHit
QueryHit
Scalability:
limited scope
flooding
2: Application Layer
15
Gnutella: Peer joining
joining peer Alice must find another peer in
Gnutella network: use list of candidate peers
2. Alice sequentially attempts TCP connections with
candidate peers until connection setup with Bob
3. Flooding: Alice sends Ping message to Bob; Bob
forwards Ping message to his overlay neighbors
(who then forward to their neighbors….)
 peers receiving Ping message respond to Alice
with Pong message
4. Alice receives many Pong messages, and can then
setup additional TCP connections
Peer leaving: see homework problem!
1.
2: Application Layer
16
Distributed Hash Table (DHT)
 DHT: a distributed P2P database
 database has (key, value) pairs; examples:
 key:
ss number; value: human name
 key: movie title; value: IP address
 Distribute the (key, value) pairs over the
(millions of peers)
 a peer queries DHT with key
 DHT
returns values that match the key
 peers can also insert (key, value) pairs
Application 2-17
Q: how to assign keys to peers?
 central issue:
 assigning
(key, value) pairs to peers.
 basic idea:
 convert
each key to an integer
 Assign integer to each peer
 put (key,value) pair in the peer that is closest
to the key
Application 2-18
DHT identifiers
 assign integer identifier to each peer in range
[0,2n-1] for some n.
 each
identifier represented by n bits.
 require each key to be an integer in same range
 to get integer key, hash original key
 e.g.,
key = hash(“Led Zeppelin IV”)
 this is why its is referred to as a distributed “hash”
table
Application 2-19
Assign keys to peers
 rule: assign key to the peer that has the
closest ID.
 convention in lecture: closest is the
immediate successor of the key.
 e.g., n=4; peers: 1,3,4,5,8,10,12,14;
 key
= 13, then successor peer = 14
 key = 15, then successor peer = 1
Application 2-20
Circular DHT (1)
1
3
15
4
12
5
10
8
 each peer only aware of immediate successor and
predecessor.
 “overlay network”
Application 2-21
Circular DHT (1)
O(N) messages
on avgerage to resolve
query, when there
are N peers
I am
0001
Who’s responsible
for key 1110 ?
0011
1111
1110
0100
1110
1110
1100
1110
1110
Define closest
as closest
successor
0101
1110
1010
1000
Application 2-22
Circular DHT with shortcuts
1
3
Who’s responsible
for key 1110?
15
4
12
5
10
8
 each peer keeps track of IP addresses of predecessor,
successor, short cuts.
 reduced from 6 to 2 messages.
 possible to design shortcuts so O(log N) neighbors, O(log N)
messages in query
Application 2-23
Peer churn
handling peer churn:
1
peers may come and go (churn)
each peer knows address of its
3
15
4
12
5
10
two successors
each peer periodically pings its
two successors to check aliveness
if immediate successor leaves,
choose next successor as new
immediate successor
8
example: peer 5 abruptly leaves
peer 4 detects peer 5 departure; makes 8 its immediate
successor; asks 8 who its immediate successor is; makes
8’s immediate successor its second successor.
what if peer 13 wants to join?
Application 2-24
Chapter 2: Application layer
 2.1 Principles of
network applications
 2.2 Web and HTTP
 2.3 FTP
 2.4 Electronic Mail

 2.6 P2P applications
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
SMTP, POP3, IMAP
 2.5 DNS
2: Application Layer
25
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
 reliable, byte streamoriented
socket
a host-local,
application-created,
OS-controlled interface
(a “door”) into which
application process can
both send and
receive messages to/from
another application
process
2: Application Layer
26
Socket-programming using TCP
Socket: a door between application process and endend-transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one
process to another
controlled by
application
developer
controlled by
operating
system
process
process
socket
TCP with
buffers,
variables
host or
server
internet
socket
TCP with
buffers,
variables
controlled by
application
developer
controlled by
operating
system
host or
server
2: Application Layer
27
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)
application viewpoint
TCP provides reliable, in-order
transfer of bytes (“pipe”)
between client and server
2: Application Layer
28
Client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
TCP
wait for incoming
connection request connection
connectionSocket =
welcomeSocket.accept()
read request from
connectionSocket
write reply to
connectionSocket
close
connectionSocket
setup
create socket,
connect to hostid, port=x
clientSocket =
Socket()
send request using
clientSocket
read reply from
clientSocket
close
clientSocket
2: Application Layer
29
Stream jargon
keyboard
monitor
output
stream
inFromServer
Client
Process
process
input
stream
outToServer
characters that flow into
or out of a process.
 An input stream is
attached to some input
source for the process,
e.g., keyboard or socket.
 An output stream is
attached to an output
source, e.g., monitor or
socket.
inFromUser
 A stream is a sequence of
input
stream
client
TCP
clientSocket
socket
to network
TCP
socket
from network
2: Application Layer
30
Socket programming with TCP
Example client-server app:
1) client reads line from
standard input (inFromUser
stream) , sends to server via
socket (outToServer
stream)
2) server reads line from socket
3) server converts line to
uppercase, sends back to
client
4) client reads, prints modified
line from socket
(inFromServer stream)
2: Application Layer
31
Example: Java client (TCP)
import java.io.*;
import java.net.*;
class TCPClient {
public static void main(String argv[]) throws Exception
{
String sentence;
String modifiedSentence;
Create
input stream
Create
client socket,
connect to server
Create
output stream
attached to socket
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Socket clientSocket = new Socket("hostname", 6789);
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
2: Application Layer
32
Example: Java client (TCP), cont.
Create
input stream
attached to socket
BufferedReader inFromServer =
new BufferedReader(new
InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
Send line
to server
outToServer.writeBytes(sentence + '\n');
Read line
from server
modifiedSentence = inFromServer.readLine();
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close();
}
}
2: Application Layer
33
Example: Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
Create
welcoming socket
at port 6789
Wait, on welcoming
socket for contact
by client
Create input
stream, attached
to socket
public static void main(String argv[]) throws Exception
{
String clientSentence;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient =
new BufferedReader(new
InputStreamReader(connectionSocket.getInputStream()));
2: Application Layer
34
Example: Java server (TCP), cont
Create output
stream, attached
to socket
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
Read in line
from socket
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + '\n';
Write out line
to socket
outToClient.writeBytes(capitalizedSentence);
}
}
}
End of while loop,
loop back and wait for
another client connection
2: Application Layer
35
Chapter 2: Application layer
 2.1 Principles of
network applications
 2.2 Web and HTTP
 2.3 FTP
 2.4 Electronic Mail

 2.6 P2P applications
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
SMTP, POP3, IMAP
 2.5 DNS
2: Application Layer
36
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
application viewpoint
UDP provides unreliable transfer
of groups of bytes (“datagrams”)
between client and server
UDP: transmitted data may be
received out of order, or
lost
2: Application Layer
37
Client/server socket interaction: UDP
Server (running on hostid)
create socket,
port= x.
serverSocket =
DatagramSocket()
read datagram from
serverSocket
write reply to
serverSocket
specifying
client address,
port number
Client
create socket,
clientSocket =
DatagramSocket()
Create datagram with server IP and
port=x; send datagram via
clientSocket
read datagram from
clientSocket
close
clientSocket
2: Application Layer
38
Example: Java client (UDP)
input
stream
Client
process
monitor
inFromUser
keyboard
Process
Input: receives
packet (recall
thatTCP received
“byte stream”)
UDP
packet
receivePacket
packet (recall
that TCP sent
“byte stream”)
sendPacket
Output: sends
UDP
packet
client
UDP
clientSocket
socket
to network
UDP
socket
from network
2: Application Layer
39
Example: Java client (UDP)
import java.io.*;
import java.net.*;
Create
input stream
Create
client socket
Translate
hostname to IP
address using DNS
class UDPClient {
public static void main(String args[]) throws Exception
{
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DatagramSocket clientSocket = new DatagramSocket();
InetAddress IPAddress = InetAddress.getByName("hostname");
byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
2: Application Layer
40
Example: Java client (UDP), cont.
Create datagram
with data-to-send,
length, IP addr, port
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
Send datagram
to server
clientSocket.send(sendPacket);
Read datagram
from server
clientSocket.receive(receivePacket);
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
2: Application Layer
41
Example: Java server (UDP)
import java.io.*;
import java.net.*;
Create
datagram socket
at port 9876
class UDPServer {
public static void main(String args[]) throws Exception
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
Create space for
received datagram
Receive
datagram
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
serverSocket.receive(receivePacket);
2: Application Layer
42
Example: Java server (UDP), cont
String sentence = new String(receivePacket.getData());
Get IP addr
port #, of
sender
InetAddress IPAddress = receivePacket.getAddress();
int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
Create datagram
to send to client
DatagramPacket sendPacket =
new DatagramPacket(sendData, sendData.length, IPAddress,
port);
Write out
datagram
to socket
serverSocket.send(sendPacket);
}
}
}
End of while loop,
loop back and wait for
another datagram
2: Application Layer
43
Chapter 2: Summary
our study of network apps now complete!
 application architectures
 client-server
 P2P
 hybrid
 application service
requirements:

reliability, bandwidth,
delay
 specific protocols:
 HTTP
 FTP
 SMTP, POP, IMAP
 DNS
 P2P: BitTorrent, Skype
 socket programming
 Internet transport
service model


connection-oriented,
reliable: TCP
unreliable, datagrams: UDP
2: Application Layer
44
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
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”
2: Application Layer
45