Objectives
• Understand
– Service requirements applications placed on network
infrastructure
– Protocols distributed applications use to implement applications
• Conceptual + implementation aspects of network
application protocols
– client server paradigm
– peer-to-peer paradigm
• Learn about protocols by examining popular
application-level protocols
– World Wide Web
– Electronic Mail
– P2P File Sharing
• Application Infrastructure Services: DNS
CSci4211:
Application Layer
1
Some network apps
•
•
•
•
•
•
e-mail
web
instant messaging
remote login
P2P file sharing
multi-user network
games
• streaming stored video
clips
CSci4211:
• social networks
• voice over IP
• real-time video
conferencing
• grid computing
Application Layer
2
Creating a network app
application
transport
network
data link
physical
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
CSci4211:
Application Layer
application
transport
network
data link
physical
application
transport
network
data link
physical
3
Applications and Application-Layer Protocols
Application: communicating,
distributed processes
– running in network hosts in
“user space”
– exchange messages to
implement app
– e.g., email, file transfer, the
Web
application
transport
network
data link
physical
Application-layer protocols
– one “piece” of an app
– define messages exchanged
by apps and actions taken
– user services provided by
lower layer protocols
CSci4211:
application
transport
network
data link
physical
Application Layer
application
transport
network
data link
physical
4
How two applications on two
different computers communicate?
CSci4211:
Application Layer
5
Analogy: Postal Service
CSci4211:
Application Layer
6
Step 1: Find out the machine
Internet Protocol (IP)
200 Union Street SE
Minneapolis, MN
CSci4211:
Application Layer
7
Addressing Machines (Hosts)
• Remembering IP
addresses is a pain in the
neck (for humans)
• To receive messages,
each machine (e.g., a web
or a desktop/laptop) must • Host (or domain) names
an “address”
– e.g., mail.cs.umn.edu, or
www.google.com
• host device has unique
– DNS translates domain
32-bit IP(v4) address
names to IP addresses
• Exercise:
– On Windows, use ipconfig • Given the IP address,
from command prompt to
Network performs
get your IP address
routing & forwarding to
– On Mac, use ifconfig
deliver msgs between
from command prompt to
(end) hosts
get your IP address
CSci4211:
Application Layer
8
IP Addresses
• Used to identify machines (network
interfaces)
• Each IP address is 32-bit
– IPv6 addresses are 128-bit
• Represented as x1.x2.x3.x4
– Each xi corresponds to a byte
– E.g.: 192.168.200.10
• Each IP packet contains a destination IP
address
CSci4211:
Application Layer
9
Hostnames
• 206.207.85.33 67.99.176.30
• www.home.com www.funnymovies.com
• Machines are good at remembering numbers, while
human beings are good at remember names.
• The name (e.g., www.cs.umn.edu) consists of multiple
parts:
– First part is a machine name (or special identifier like www)
– Each successive part is a domain name which contains the
previous domain
CSci4211:
Application Layer
10
Domain Name Service (DNS)
• IP routing uses IP addresses
• Need a way to convert hostnames to IP
addresses
• DNS is a distributed mapping service
– Maintains “table” of name-to-address mapping
– Used by most applications. E.g.: Web, email, etc.
• Advantages
– Easier for programmers and users
– Can change mapping if needed
– more next week …..
CSci4211:
Application Layer
11
Internet Routing
• The Internet consists of a number of routers
• Each router forwards packets onto the next
hop
• Goal is to move the packet closer to its
destination
– Each router has a table
– Matches packet address to determine next hop
CSci4211:
Application Layer
12
Step 2: Find out the process
Transport layer Protocol
CSci4211:
Application Layer
13
Addressing Processes
• Q: does IP address of
• to receive messages,
host on which process runs
process must have
suffice for identifying the
identifier
process?
– A: No, many processes
• host device has unique
can be running on same
32-bit IPv4 address
• Identifier includes both IP
• Exercise:
– On Windows, use ipconfig address and port numbers
associated with process on
from command prompt to
host.
get your IP address
• Example port numbers:
– On Mac, use ifconfig
from command prompt to
– HTTP server: 80
get your IP address
– Mail server: 25
CSci4211:
Application Layer
14
Identifying Remote Processes
• IP addresses and hostnames allow you to
identify machines
• But what about processes on these
machines?
• Can we use PIDs?
CSci4211:
Application Layer
15
Ports
• Identifiers for remote processes
• Each application communicates using a port
• Communication is addressed to a port on a
machine
– Delivers the packets to the process using the port
• Both TCP and UDP have their own port
numbers
• Many applications use well-known port
numbers
– HTTP: 80, FTP: 21
16
Analogy
House address: name Vs.
IP address: Port number
Bob
200 Union Street SE
Minneapolis, MN
CSci4211:
Application Layer
17
Summary: to communicate
• Sender shall include both IP address and port
numbers associated with process on host.
• Example port numbers:
– HTTP server: 80
– Mail server: 25
• For example, to send HTTP message to
gaia.cs.umass.edu web server:
– IP address: 128.119.245.12
– Port number: 80
• more shortly…
CSci4211:
Application Layer
18
Step 3: What kind of service
you need
Transport layer Protocol
CSci4211:
Application Layer
19
Network Transport Services
end host to end host communication services
• Connection-Oriented, Reliable Service
– Mimic “dedicated link”
– Messages delivered in correct order, without errors
– Transport service aware of connection in progress
• Stateful, some “state” information must be maintained
– Require explicit connection setup and teardown
• Connectionless, Unreliable Service
– Messages treated as independent
– Messages may be lost, or delivered out of order
– No connection setup or teardown, “stateless”
CSci4211:
Application Layer
20
Internet Transport Protocols
UDP service:
TCP service:
• connection-oriented: setup
required between client,
server
• reliable transport between
sender and receiver
• flow control: sender won’t
overwhelm receiver
• congestion control: throttle
sender when network
overloaded
CSci4211:
• unreliable data
transfer between
sender and receiver
• does not provide:
connection setup,
reliability, flow
control, congestion
control
Q:Why UDP?
Application Layer
21
What transport service does an app need?
Data loss
• some apps (e.g., audio) can
tolerate some loss
• other apps (e.g., file
transfer, telnet) require
100% reliable data
transfer
Timing
• some apps (e.g.,
Internet telephony,
interactive games)
require low delay to be
“effective”
CSci4211:
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, …
Application Layer
22
Transport service requirements of common apps
Application
file transfer
e-mail
Web documents
real-time
audio/video
Data loss
Bandwidth
Time Sensitive
no loss
no loss
loss-tolerant
loss-tolerant
elastic
elastic
elastic
audio: 5Kb-1Mb
video:10Kb-5Mb
same as above
few Kbps up
elastic
no
no
no
yes, 100’s msec
stored audio/video loss-tolerant
interactive games loss-tolerant
Instant messaging no loss
CSci4211:
Application Layer
yes, few secs
yes, 100’s msec
yes and no
23
Internet apps: their protocols and transport
protocols
Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia
remote file server
Internet telephony
Application
layer protocol
Underlying
transport protocol
smtp [RFC 821]
telnet [RFC 854]
http [RFC 2068]
ftp [RFC 959]
proprietary
(e.g. RealNetworks)
NSF
proprietary
(e.g., Vocaltec)
TCP
TCP
TCP
TCP
TCP or UDP
CSci4211:
Application Layer
TCP or UDP
typically UDP
24
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
 Note: applications with P2P
architectures have client
processes & server
processes
Application Layer
25
Network Applications: some jargon
• A process is a program
that is running within a
host.
• Within the same host, two
processes communicate
with interprocess
communication defined by
the OS.
• Processes running in
different hosts
communicate with an
application-layer protocol
CSci4211:
• A user agent is an
interface between the
user and the network
application.
– Web: browser
– E-mail: mail reader
– streaming audio/video:
media player
Application Layer
26
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
CSci4211:
Public-domain protocols:
• defined in RFCs
• allows for
interoperability
• e.g., HTTP, SMTP,
BitTorrent
Proprietary protocols:
• e.g., Skype, ppstream
Application Layer
2: Application Layer
27
Application Programming Interface
Q: how does a process
API: application
“identify” the other
programming interface
process with which it
• defines interface
wants to communicate?
between application
– IP address of host running
and transport layer
other process
– “port number” - allows
• socket: Internet API
– two processes
communicate by sending
data into socket, reading
data out of socket
receiving host to
determine to which local
process the message
should be delivered
 API: (1) choice of transport protocol; (2) ability to fix a few
parameters (lots more on this later)
CSci4211:
Application Layer
28
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 which
brings message to socket
at receiving process
CSci4211:
host or
server
host or
server
controlled by
app developer
process
process
socket
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
controlled
by OS
Application Layer
2: Application Layer
29
Application Structure
Internet applications distributed in nature!
- Set of communicating application-level processes
(usually on different hosts) provide/implement services
Programming Paradigms:
• Client-Server Model: Asymmetric
– Server: offers service via well defined “interface”
– Client: request service
– Example: Web; cloud computing
• Peer-to-Peer: Symmetric
– Each process is an equal
– Example: telephone, p2p file sharing (e.g., Kazaar)
• Hybrid of client-server and P2P
All require transport of “request/reply”, sharing of data!
CSci4211:
Application Layer
30
Client-server architecture
server:
– always-on host
– permanent IP address
– server farms for scaling
clients:
client/server
– communicate with server
– may be intermittently
connected
– may have dynamic IP
addresses
– do not communicate
directly with each other
2: Application Layer
31
Google Data Centers
• Estimated cost of data center: $600M
• Google spent $2.4B in 2007 on new data
centers
• Each data center uses 50-100 megawatts
of power
CSci4211:
Application Layer
32
Pure P2P architecture
• no always-on server
• arbitrary end systems
directly communicate peer-peer
• peers are intermittently
connected and change IP
addresses
Highly scalable but
difficult to manage
2: Application Layer
33
Peer-to-Peer Paradigm
• How do we implement peer-to-peer model?
• Is email peer-to-peer or client-server application?
• How do we implement peer-to-peer using
client-server model?
Difficulty in implementing “pure” peer-to-peer
model?
• How to locate your peer?
– Centralized “directory service:” i.e., white pages
• Napters
– Unstructured: e.g., “broadcast” your query: namely, ask your
friends/neighbors, who may in turn ask their friends/neighbors,
• Freenet
– Structured: Distributed hashing table (DHT)
CSci4211:
Application Layer
34
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
CSci4211:
Application Layer
2: Application Layer
35
Client-Server Paradigm Recap
Typical network app has two
pieces: client and server
Client:
•
•
•
initiates contact with server
(“speaks first”)
typically requests service from
server,
for Web, client is implemented in
browser; for e-mail, in mail reader
application
transport
network
data link
physical
request
Server:
• provides requested service to
client
• e.g., Web server sends
requested Web page, mail
server delivers e-mail
CSci4211:
Application Layer
reply
application
transport
network
data link
physical
36
Client-Server: The Web Example
some jargon
• Web page:
– consists of “objects”
– addressed by a URL
• Most Web pages
consist of:
– base HTML page, and
– several referenced
objects.
• URL has two
components: host name
and path name:
• User agent for Web is
called a browser:
– MS Internet Explorer
– Netscape Communicator
• Server for Web is
called Web server:
– Apache (public domain)
– MS Internet Information
Server
www.someSchool.edu/someDept/pic.gif
CSci4211:
Application Layer
37
The Web: the HTTP protocol
HTTP: hypertext transfer
protocol
• Web’s application layer
protocol
• client/server model
– client: browser that
requests, receives,
“displays” Web objects
– server: Web server
sends objects in
response to requests
•
•
http1.0: RFC 1945
http1.1: RFC 2068
PC running
Explorer
Server
running
NCSA Web
server
Mac running
Navigator
CSci4211:
Application Layer
38
The HTTP protocol: more
http: TCP transport
service:
http is “stateless”
• client initiates TCP
connection (creates socket)
to server, port 80
• server accepts TCP
connection from client
• http messages (applicationlayer protocol messages)
exchanged between browser
(http client) and Web server
(http server)
• TCP connection closed
CSci4211:
• server maintains no
information about
past client requests
Protocols that maintain aside
“state” are complex!
• past history (state) must
be maintained
• if server/client crashes,
their views of “state” may
be inconsistent, must be
reconciled
Application Layer
39
HTTP Example
Suppose user enters URL
www.someSchool.edu/someDepartment/home.index
(contains text, references to 10 jpeg images)
1a. http client initiates TCP
connection to http server
(process) at
www.someSchool.edu. Port 80 is
default for http server.
2. http client sends http request
message (containing URL) into
TCP connection socket
time
CSci4211:
1b. http server at host
www.someSchool.edu waiting
for TCP connection at port 80.
“accepts” connection, notifying
client
3. http server receives request
message, forms response
message containing requested
object
(someDepartment/home.index),
sends message into socket
Application Layer
40
HTTP Example (cont.)
4. http server closes TCP
connection.
5. http client receives
response message
containing html file,
displays html. Parsing html
file, finds 10 referenced
jpeg objects
6. Steps 1-5 repeated for each
of 10 jpeg objects
time
CSci4211:
Application Layer
41
Non-persistent and persistent connections
Non-persistent
• HTTP/1.0
• server parses request,
responds, and closes TCP
connection
• 2 RTTs to fetch each
object
• Each object transfer
suffers from slow start
But most 1.0 browsers use
parallel TCP connections.
CSci4211:
Persistent
• default for HTTP/1.1
• on same TCP connection:
server, parses request,
responds, parses new
request,..
• Client sends requests for
all referenced objects as
soon as it receives base
HTML.
• Fewer RTTs and less slow
start.
Application Layer
42
http message format: request
• two types of http messages: request, response
• http request message:
– ASCII (human-readable format)
request line
(GET, POST,
HEAD commands)
GET /somedir/page.html HTTP/1.0
User-agent: Mozilla/4.0
header Accept: text/html, image/gif,image/jpeg
lines Accept-language:fr
Carriage return,
line feed
(extra carriage return, line feed)
indicates end
of message
CSci4211:
Application Layer
43
http request message: general format
CSci4211:
Application Layer
44
http message format: response
status line
(protocol
status code
status phrase)
header
lines
data, e.g.,
requested
html file
HTTP/1.0 200 OK
Date: Thu, 06 Aug 1998 12:00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html
data data data data data ...
CSci4211:
Application Layer
45
http response status codes
In first line in server->client response message.
A few sample codes:
200 OK
– request succeeded, requested object later in this message
301 Moved Permanently
– requested object moved, new location specified later in this
message (Location:)
400 Bad Request
– request message not understood by server
404 Not Found
– requested document not found on this server
505 HTTP Version Not Supported
CSci4211:
Application Layer
46
Trying out http (client side) for yourself
1. Telnet to your favorite Web server:
telnet www.eurecom.fr 80 Opens TCP connection to port 80
(default http server port) at www.eurecom.fr.
Anything typed in sent
to port 80 at www.eurecom.fr
2. Type in a GET http request:
GET /~ross/index.html HTTP/1.0
By typing this in (hit carriage
return twice), you send
this minimal (but complete)
GET request to http server
3. Look at response message sent by http server!
CSci4211:
Application Layer
47
Web and HTTP Summary
Transaction-oriented (request/reply), use TCP, port 80
Client
Server
GET /index.html HTTP/1.0
CSci4211:
HTTP/1.0
200 Document follows
Content-type: text/html
Content-length: 2090
-- blank line -HTML text of the Web page
Application Layer
48
User-server interaction: authentication
Authentication goal: control
access to server documents
• stateless: client must present
authorization in each request
• authorization: typically name,
password
client
usual http request msg
401: authorization req.
WWW authenticate:
usual http request msg
+ Authorization:line
– authorization: header line
in request
– if no authorization
presented, server refuses
access, sends
usual http response msg
WWW authenticate:
usual http request msg
+ Authorization:line
header line in response
Browser caches name & password so
that user does not have to repeatedly enter it.
CSci4211:
server
usual http response msg
Application Layer
time
49
User-server interaction: cookies
• server sends “cookie” to
client in response mst
server
client
Set-cookie: 1678453
• client presents cookie in
later requests
cookie: 1678453
• server matches
presented-cookie with
server-stored info
– authentication
– remembering user
preferences, previous
choices
CSci4211:
usual http request msg
usual http response +
Set-cookie: #
usual http request msg
cookie: #
usual http response msg
usual http request msg
cookie: #
usual http response msg
Application Layer
cookiespeccific
action
cookiespecific
action
50
Electronic Mail
outgoing
message queue
user mailbox
Three major components:
• user agents
• mail servers
• simple mail transfer
protocol: smtp
user
agent
mail
server
user
agent
SMTP
User Agent
SMTP
• a.k.a. “mail reader”
• composing, editing, reading
mail
mail messages
server
• e.g., Eudora, Outlook, pine,
Netscape Messenger
• outgoing, incoming messages
stored on server
user
SMTP
mail
server
user
agent
user
agent
user
agent
agent
CSci4211:
Application Layer
51
Electronic Mail: mail servers
user
agent
Mail Servers
• mailbox contains incoming
messages (yet to be read)
for user
• message queue of outgoing
(to be sent) mail messages
• smtp protocol between mail
servers to send email
messages
– client: sending mail server
– “server”: receiving mail
server
mail
server
user
agent
SMTP
SMTP
SMTP
mail
server
mail
server
user
agent
user
agent
user
agent
user
agent
CSci4211:
Application Layer
52
Electronic Mail:SMTP [RFC 821]
• uses tcp to reliably transfer email msg from client to
server, port 25
• direct transfer: sending server to receiving server
• three phases of transfer
– handshaking (greeting)
– transfer of messages
– closure
• command/response interaction
– commands: ASCII text
– response: status code and phrase
• messages must be in 7-bit ASCII
CSci4211:
Application Layer
53
Sample SMTP Interaction
S:
C:
S:
C:
S:
C:
S:
C:
S:
C:
C:
C:
S:
C:
S:
220 hamburger.edu
HELO crepes.fr
250 Hello crepes.fr, pleased to meet you
MAIL FROM: <alice@crepes.fr>
250 alice@crepes.fr... Sender ok
RCPT TO: <bob@hamburger.edu>
250 bob@hamburger.edu ... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
Do you like ketchup?
How about pickles?
.
250 Message accepted for delivery
QUIT
221 hamburger.edu closing connection
CSci4211:
Application Layer
54
Try SMTP interaction yourself
• telnet servername 25
• see 220 reply from server
• enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
commands
above lets you send email without using email client
(reader)
CSci4211:
Application Layer
55
SMTP: final words
• smtp uses persistent
connections
• smtp requires that
message (header & body)
be in 7-bit ascii
• certain character strings
are not permitted in
message (e.g., CRLF.CRLF).
Thus message has to be
encoded (usually into either
base-64 or quoted
printable)
• smtp server uses
CRLF.CRLF to determine
end of message
CSci4211:
Comparison with http
• http: pull
• email: push
• both have ASCII
command/response
interaction, status codes
• http: each object is
encapsulated in its own
response message
• smtp: multiple objects
message sent in a multipart
message
Application Layer
56
Mail message format
smtp: protocol for exchanging
email msgs
RFC 822: standard for text
message format:
• header lines, e.g.,
header
blank
line
body
– To:
– From:
– Subject:
different from smtp commands!
• body
– the “message”, ASCII
characters only
CSci4211:
Application Layer
57
Message format: multimedia extensions
• MIME: multimedia mail extension, RFC 2045, 2056
• additional lines in msg header declare MIME content
type
From: alice@crepes.fr
To: bob@hamburger.edu
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
MIME version
method used
to encode data
multimedia data
type, subtype,
parameter declaration
base64 encoded data .....
.........................
......base64 encoded data
encoded data
CSci4211:
Application Layer
58
MIME types
Content-Type: type/subtype; parameters
Text
• example subtypes: plain,
html
Image
• example subtypes: jpeg,
gif
Audio
• example subtypes: basic
(8-bit mu-law encoded),
32kadpcm (32 kbps
coding)
CSci4211:
Video
• example subtypes: mpeg,
quicktime
Application
• other data that must be
processed by reader
before “viewable”
• example subtypes:
msword, octet-stream
Application Layer
59
Multipart Type
From: alice@crepes.fr
To: bob@hamburger.edu
Subject: Picture of yummy crepe.
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=98766789
--98766789
Content-Transfer-Encoding: quoted-printable
Content-Type: text/plain
Dear Bob,
Please find a picture of a crepe.
--98766789
Content-Transfer-Encoding: base64
Content-Type: image/jpeg
base64 encoded data .....
.........................
......base64 encoded data
--98766789--
CSci4211:
Application Layer
60
Mail access protocols
SMTP
SMTP
user
agent
sender’s mail
server
•
•
POP3 or
IMAP
user
agent
receiver’s mail
server
SMTP: delivery/storage to receiver’s server
Mail access protocol: retrieval from server
– POP: Post Office Protocol [RFC 1939]
• authorization (agent <-->server) and download
– IMAP: Internet Mail Access Protocol [RFC 1730]
• more features (more complex)
• manipulation of stored msgs on server
– HTTP: Hotmail , Yahoo! Mail, etc.
CSci4211:
Application Layer
61
POP3 protocol
authorization phase
• client commands:
– user: declare username
– pass: password
• server responses
– +OK
– -ERR
transaction phase,
client:
• list: list message numbers
• retr: retrieve message by
number
• dele: delete
• quit
CSci4211:
S:
C:
S:
C:
S:
+OK POP3 server ready
user alice
+OK
pass hungry
+OK user successfully logged
C:
S:
S:
S:
C:
S:
S:
C:
C:
S:
S:
C:
C:
S:
list
1 498
2 912
.
retr 1
<message 1 contents>
.
dele 1
retr 2
<message 1 contents>
.
dele 2
quit
+OK POP3 server signing off
Application Layer
62
on
Email Summary
Alice
Message
user agent
(MUA)
client
Message
transfer
agent
(MTA)
SMTP
outgoing mail queue
SMTP
over TCP
(RFC 821)
Bob
POP3 (RFC 1225)/ IMAP (RFC 1064)
for accessing mail
Message
user agent
(MUA)
user mailbox
CSci4211:
Application Layer
port 25
server
Message
transfer
agent
(MTA)
63
Application Layer
•
•
•
•
World Wide Web
Electronic Mail
Domain Name System
P2P File Sharing
Readings: Chapter 2: section 2.1-2.6
CSci4211:
Application Layer
64
Internet: Naming and Addressing
• Names, addresses and routes:
According to Shoch (1979)
– name: identifies what you want
– address: identifies where it is
– route: identifies a way to get there
• Internet names and addresses
Example
Organization
flat, permanent
IP address
128.101.35.34
2-level
Host name
afer.cs.umn.edu
hierarchical
MAC address
CSci4211:
Application Layer
65
IP addresses
• Two-level hierarchy: network id. + host id.
• (or rather 3-level, subnetwork id.)
– 32 bits long usually written in dotted decimal notation
e.g., 128.101.35.34
• No two hosts have the same IP address
• host’s IP address may change, e.g., dial-in hosts
– a host may have multiple IP addresses
– IP address identifies host interface
• Mapping of IP address to MAC (physical) IP done
using IP ARP (this is called address resolution)
• one-to-one mapping
• Mapping between IP address and host name done
using Domain Name Servers (DNS)
•
many-to-many mapping
CSci4211:
Application Layer
66
Internet Domain Names
• Hierarchical: anywhere
. (root)
from two to possibly
infinity
• Examples:
afer.cs.umn.edu,
lupus.fokus.gmd.de
edu, de: organization type
or country (a “domain”)
– umn, fokus: organization
administering the “subdomain”
– cs, fokus: organization
administering the host
– afer, lupus: host name (have
IP address)
. com
. uk
. edu
–
CSci4211:
umn.edu
yahoo.com
cs.umn.edu
itlabs.umn.edu
www.yahoo.com
afer.cs.umn.edu
Application Layer
67
Domain Name Resolution and DNS
DNS: Domain Name System:
• distributed database
implemented in hierarchy of
many name servers
• application-layer protocol host,
routers, name servers to
communicate to resolve names
(address/name translation)
– note: core Internet function
implemented as application-layer
protocol
– complexity at network’s “edge”
CSci4211:
• hierarchy of redundant
servers with time-limited
cache
• 13 root servers, each
knowing the global top-level
domains (e.g., edu, gov, com)
, refer queries to them
• each server knows the 13
root servers
• each domain has at least 2
servers (often widely
distributed) for fault
distributed
• DNS has info about other
resources, e.g., mail servers
Application Layer
68
DNS name servers
Why not centralize DNS?
• single point of failure
• traffic volume
• distant centralized
database
• maintenance
• no server has all nameto-IP address mappings
local name servers:
– each ISP, company has local
(default) name server
– host DNS query first goes to
local name server
authoritative name server:
– for a host: stores that host’s
IP address, name
– can perform name/address
translation for that host’s
name
doesn’t scale!
CSci4211:
Application Layer
69
DNS: Root name servers
• contacted by local
name server that can
not resolve name
• root name server:
– contacts
authoritative name
server if name
mapping not known
– gets mapping
– returns mapping to
local name server
• ~ dozen root name
servers worldwide
CSci4211:
Application Layer
70
Simple DNS example
host homeboy.aol.com
wants IP address of
afer.cs.umn.edu
root name server
2
4
5
3
1. Contacts its local DNS
server, dns.aol.com
local name server authorititive name server
dns.aol.com
2. dns.aol.com contacts
dns.umn.edu
root name server, if
1
6
necessary
3. root name server contacts
authoritative name server,
dns.umn.edu, if
requesting host
afer.cs.umn.com
homeboy.aol.com
necessary
CSci4211:
Application Layer
71
root name server
DNS example
Root name server:
6
2
• may not know
authoritative name
server
• may know
intermediate name
server: who to
contact to find
authoritative name
server
7
3
local name server intermediate name server
dns.aol.com
1
8
dns.umn.edu.
4
5
authoritative name server
requesting host
dns.cs.umn.edu
homeboy.aol.com
afer.cs.umn.edu
CSci4211:
Application Layer
72
DNS: iterated queries
recursive query:
root name server
• puts burden of name
resolution on
contacted name
server
• heavy load?
3
4
7
local name server intermediate name server
dns.aol.com
iterated query:
• contacted server
replies with name of
server to contact
• “I don’t know this
name, but ask this
server”
iterated query
2
1
8
dns.umn.edu
5
6
authoritative name server
requesting host
dns.cs.umn.edu
homeboy.aol.com
CSci4211:
afer.cs.umass.edu
Application Layer
73
DNS: caching and updating records
• once (any) name server learns mapping, it caches
mapping
– cache entries timeout (disappear) after some time
• update/notify mechanisms under design by IETF
– RFC 2136
– http://www.ietf.org/html.charters/dnsind-charter.html
CSci4211:
Application Layer
74
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name,
value, type,ttl)
• Type=CNAME
• Type=A
– name is hostname
– value is IP address
• Type=NS
– name is domain (e.g.
foo.com)
– value is IP address of
authoritative name server
for this domain
CSci4211:
– name is an alias name for
some “canonical” (the real)
name
– value is canonical name
• Type=MX
– value is hostname of mailserver
associated with name
Application Layer
75
DNS protocol, messages
DNS protocol : query and reply messages, both with same
message format
msg header
• identification: 16 bit # for
query, reply to query uses
same #
• flags:
–
–
–
–
query or reply
recursion desired
recursion available
reply is authoritative
CSci4211:
Application Layer
76
DNS protocol, messages
Name, type fields
for a query
RRs in reponse
to query
records for
authoritative servers
additional “helpful”
info that may be used
CSci4211:
Application Layer
77
DNS Protocol
• Query/Reply: use UDP
• Transfer of DNS Records between
authoritative and replicated servers:
use TCP
CSci4211:
Application Layer
78
P2P File Sharing
Example
• Alice runs P2P client
application on her notebook
computer
• Intermittently connects to
Internet; gets new IP
address for each
connection
• Asks for “Hey Jude”
• Application displays other
peers that have copy of
Hey Jude.
CSci4211:
• Alice chooses one of the
peers, Bob.
• File is copied from Bob’s PC
to Alice’s notebook: HTTP
• While Alice downloads,
other users uploading from
Alice.
• Alice’s peer is both a Web
client and a transient Web
server.
All peers are servers = highly
scalable!
Application Layer
79
P2P: Centralized Directory
Bob
original “Napster” design
1) when peer connects, it
informs central server:
centralized
directory server
1
peers
1
– IP address
– content
3
1
2) Alice queries for “Hey
Jude”
3) Alice requests file from
Bob
2
1
Alice
CSci4211:
Application Layer
80
P2P: problems with centralized directory
• Single point of failure
• Performance
bottleneck
• Copyright
infringement
CSci4211:
file transfer is
decentralized, but
locating content is
highly centralized
Application Layer
81
Query Flooding: Gnutella
• fully distributed
– no central server
• public domain protocol
• many Gnutella clients
implementing protocol
CSci4211:
overlay network: graph
• edge between peer X
and Y if there’s a TCP
connection
• all active peers and
edges is overlay net
• Edge is not a physical
link
• Given peer will
typically be connected
with < 10 overlay
neighbors
Application Layer
82
Gnutella: protocol
 Query message
sent over existing TCP
connections
 peers forward
Query message
 QueryHit
sent over
reverse
path
File transfer:
HTTP
Query
QueryHit
Query
QueryHit
Scalability:
limited scope
flooding
CSci4211:
Application Layer
83
Gnutella: Peer Joining
1.
Joining peer X must find some other peer in
Gnutella network: use list of candidate peers
2. X sequentially attempts to make TCP with peers
on list until connection setup with Y
3. X sends Ping message to Y; Y forwards Ping
message.
4. All peers receiving Ping message respond with
Pong message
5. X receives many Pong messages. It can then
setup additional TCP connections
Peer leaving: see homework problem 16 in Textbook!
CSci4211:
Application Layer
84
BitTorrent
• Files are shared by many users (as chunks:
around 256KB)
• Active participation: peers download and
upload chunks
• A torrent is a group of peers that contain
chunks of a file.
• Each torrent has a tracker that keeps
track of participating peers
CSci4211:
Application Layer
85
CSci4211:
Application Layer
2: Application Layer
86
Torrent Setup
Tracker
p2p_1
p2p_2
Alice
p2p_3
CSci4211:
Application Layer
87
Trading chunks
• What does Alice know?
– Subset of chunks she have.
– Which chunks her neighbors have.
• Which chunks she requests first form
neighbors?
– Use rarest first (chunks with least repeated copies).
• Which requests should Alice respond to?
– Priority is given to neighbors supplying her data at the
highest rate.
– Utilize unchoked and optimistically unchocked peers.
– Tit-for-tat
CSci4211:
Application Layer
88
P2P Case study: Skype
Skype clients (SC)
• inherently P2P: pairs
of users communicate.
• proprietary
Skype
login server
application-layer
protocol (inferred via
reverse engineering)
• hierarchical overlay
with SNs
• Index maps usernames
to IP addresses;
distributed over SNs
Supernode
(SN)
2: Application Layer
89
Peers as relays
• Problem when both
Alice and Bob are
behind “NATs”.
– NAT prevents an outside
peer from initiating a call
to insider peer
• Solution:
– Using Alice’s and Bob’s
SNs, Relay is chosen
– Each peer initiates
session with relay.
– Peers can now
communicate through
NATs via relay
2: Application Layer
90
Exploiting Heterogeneity: KaZaA
• Each peer is either a
group leader or assigned
to a group leader.
– TCP connection between
peer and its group leader.
– TCP connections between
some pairs of group leaders.
• Group leader tracks the
content in all its
children.
ordinary peer
group-leader peer
neighoring relationships
in overlay network
CSci4211:
Application Layer
91
KaZaA: Querying
• Each file has a hash and a descriptor
• Client sends keyword query to its group leader
• Group leader responds with matches:
– For each match: metadata, hash, IP address
• If group leader forwards query to other group
leaders, they respond with matches
• Client then selects files for downloading
– HTTP requests using hash as identifier sent to peers
holding desired file
CSci4211:
Application Layer
92
KaZaA Tricks
•
•
•
•
Limitations on simultaneous uploads
Request queuing
Incentive priorities
Parallel downloading
For more info:
 J. Liang, R. Kumar, K. Ross, “Understanding KaZaA,”
(available via cis.poly.edu/~ross)
CSci4211:
Application Layer
93
Summary
• Application Service Requirements:
–
reliability, bandwidth, delay
• Client-server vs. Peer-to-Peer Paradigm
• Application Protocols and Their Implementation:
–
–
–
–
specific formats: header, data;
control vs. data messages
stateful vs. stateless
centralized vs. decentralized
• Specific Protocols:
– http
– smtp, pop3
– dns
CSci4211:
Application Layer
94
Optional Material
CSci4211:
Application Layer
95
Distributed Hash Table (DHT)
• DHT = distributed P2P database
• Database has (key, value) pairs;
– key: ss number; value: human name
– key: content type; value: IP address
• Peers query DB with key
– DB returns values that match the key
• Peers can also insert (key, value) peers
CSci4211:
Application Layer
96
DHT Identifiers
• Assign integer identifier to each peer in range
[0,2n-1].
– Each identifier can be represented by n bits.
• Require each key to be an integer in same range.
• To get integer keys, hash original key.
– eg, key = h(“Led Zeppelin IV”)
– This is why they call it a distributed “hash” table
CSci4211:
Application Layer
How to assign keys to peers?
• Central issue:
– Assigning (key, value) pairs to peers.
• Rule: assign key to the peer that has the
closest ID.
• Convention in lecture: closest is the
immediate successor of the key.
• Ex: n=4; peers: 1,3,4,5,8,10,12,14;
– key = 13, then successor peer = 14
– key = 15, then successor peer = 1
CSci4211:
Application Layer
Circular DHT (1)
1
3
15
4
12
5
10
8
• Each peer only aware of immediate successor
and predecessor.
• “Overlay network”
CSci4211:
Application Layer
99
Circle DHT (2)
O(N) messages
on avg to resolve
query, when there
are N peers
0001
I am
Who’s resp
for key 1110 ?
0011
1111
1110
0100
1110
1110
1100
Define closest
as closest
successor
1110
1110
0101
1110
1010
CSci4211:
1000
Application Layer
10
0
Circular DHT with Shortcuts
1
Who’s resp
for key 1110?
3
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
CSci4211:
Application Layer
101
Peer Churn
1
3
15
4
12
•To handle peer churn, require
each peer to know the IP address
of its two successors.
• Each peer periodically pings its
two successors to see if they
are still alive.
5
10
8
• Peer 5 abruptly leaves
• Peer 4 detects; 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?
CSci4211:
Application Layer
10
2