1. THE APPLICATION LAYER

2. A closer look at network structure
 network edge: applications and
hosts
 network core:
 routers
 network of networks
 access networks, physical
media: communication links

3. The network edge
run application programs e.g. Web, email at “edge of
network”
3

4. Network Application Architecture
 Client Server
 P2P
 Hybrid

5. Client-server architecture
server:
 always-on host
 permanent IP address
 server farms for scaling
 clients:
 communicate with server
 may be intermittently
connected
 may have dynamic IP
addresses
 do not communicate directly
with each other
5

6. Pure P2P architecture
 no always-on server
 arbitrary end systems
directly communicate
 peers are intermittently
connected and change IP
addresses
 example: Gnutella
Highly scalable but difficult
to manage
2: Application Layer 6

7. Hybrid of client-server and P2P
Skype
 Internet telephony app
 Finding address of remote party: centralized server(s)
 Client-client connection is direct (not through server)
 Instant messaging
 Chatting between two users is P2P
 Presence detection/location centralized:
 User registers its IP address with central server when it
comes online
 User contacts central server to find IP addresses of buddies
2: Application Layer

8. Creating a network app
Write programs that
application
 run on different end systems transport
network
and communicate over a data link
physical
network.
 e.g., Web: Web server software
communicates with browser
software
 little software written for devices
in network core application
application
transport
transport network
 network core devices do not run network data link
data link physical
user application code physical
 application on end systems
allows for rapid app

9. 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
 Note: applications with P2P architectures have client processes &
server processes
Client process: process that
initiates communication
Server process: process that
waits to be contacted

10. 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
 Socket is an interface between application layer and transport layer within a host : it
is referred as Application Programming Interface (API):
 Application developer can only control (on transport layer side)
(1) choice of transport protocol (TCP or UDP);
(2) ability to fix a few parameters such as max buffer and max segment size
10

11. Sockets
host or host or
server server
controlled by
app developer
process process
socket socket
TCP with TCP with
buffers, Internet buffers,
variables variables
controlled
by OS
11

12. Sockets
Steps followed by client to establish the connection:
1. Create a socket
2. Connect the socket to the address of the server
3. Send/Receive data
4. Close the socket
Steps followed by server to establish the connection:
1. Create a socket
2. Bind the socket to the port number known to all clients
3. Listen for the connection request
4. Accept connection request
5. Send/Receive data

13. Addressing processes
 to receive messages, process must have identifier
 host device has unique32-bit IP address
 Q: does IP address of host on which process runs suffice for
identifying the process?
 Answer: NO, many processes can be running on same host
 identifier includes both IP address and port numbers associated with
process on host.
 Example port numbers:
 HTTP server: 80
 Mail server: 25

14. App-layer protocol defines
 Types of messages exchanged, Public-domain protocols:
 defined in RFCs
 e.g., request, response
 allows for interoperability
 Message syntax:  e.g., HTTP, SMTP
 what fields in messages &  Proprietary protocols:
how fields are delineated  e.g., KaZaA
 Message semantics
 meaning of information in
fields
 Rules for when and how
processes send & respond to
messages

15. Transport service requirements of common
applications
Application Data loss Bandwidth Time Sensitive
file transfer no loss elastic no
e-mail no loss elastic no
Web documents no loss elastic no
real-time audio/video loss-tolerant audio: 5kbps-1Mbps yes, 100’s msec
video:10kbps-5Mbps
stored audio/video loss-tolerant same as above yes, few secs
interactive games loss-tolerant few kbps up yes, 100’s msec
instant messaging no loss elastic yes and no

16. Internet transport protocols services
TCP service: UDP service:
 connection-oriented: setup
 unreliable data transfer
required between client and between sending and
server processes receiving process
 reliable transport between
 does not provide:
sending and receiving process connection setup,
 flow control: sender won’t reliability, flow control,
overwhelm receiver congestion control,
 congestion control: throttle timing, or bandwidth
sender when network guarantee
overloaded
 does not provide: timing, Q: why bother? Why is there
minimum bandwidth guarantees a UDP?

17. Internet apps: application, transport protocols
Application Underlying
Application layer protocol transport protocol
e-mail SMTP [RFC 2821] TCP
remote terminal access Telnet [RFC 854] TCP
Web HTTP [RFC 2616] TCP
file transfer FTP [RFC 959] TCP
streaming multimedia proprietary TCP or UDP
(e.g. RealNetworks)
Internet telephony proprietary
(e.g., Vonage,Dialpad) typically UDP

18. Web and HTTP
 First some jargon
 Web page consists of objects
 Object can be HTML file, JPEG image, Java applet,
audio file,…
 Web page consists of base HTML-file which includes
several referenced objects
 Each object is addressable by a URL
 Example URL:
www.someschool.edu/someDept/pic.gif
host name path name

19. HTTP overview
HTTP: hypertext transfer
HT
protocol TP
req
PC running HT ues
 Web’s application layer TP t
Explorer res
pon
protocol se
 client/server model
u est
eq
 client: browser that TT
Pr
pon
se Server
H res running
requests, receives, HT
TP Apache Web
server
“displays” Web objects
 server: Web server sends Mac running
objects in response to Navigator
requests

20. HTTP overview (continued)
HTTP is “stateless”
Uses TCP:
 server maintains no
 client initiates TCP connection
information about past
(creates socket) to server, port
client requests
80
aside
 server accepts TCP connection
Protocols that maintain “state” are
from client complex!
 past history (state) must be maintained
 HTTP messages (application-
 if server/client crashes, their views of
layer protocol messages) “state” may be inconsistent, must be
exchanged between browser reconciled
(HTTP client) and Web server
(HTTP server)
 TCP connection closed

21. HTTP connections
Nonpersistent HTTP
 At most one object is sent over a TCP connection.
 HTTP/1.0 uses nonpersistent HTTP
Persistent HTTP
 Multiple objects can be sent over single TCP connection
between client and server.
 HTTP/1.1 uses persistent connections in default mode

22. Nonpersistent HTTP
Suppose user enters URL (contains text,
www.someSchool.edu/someDepartment/home.index references to 10
1a. HTTP client initiates TCP jpeg images)
connection to HTTP server (process)
at www.someSchool.edu on port 80
1b. HTTP server at host
www.someSchool.edu waiting
for TCP connection at port 80.
“accepts” connection, notifying
2. HTTP client sends HTTP client
request message (containing
URL) into TCP connection 3. HTTP server receives request
socket. Message indicates that message, forms response
client wants object message containing requested
someDepartment/home.index object, and sends message into
time its socket

23. Nonpersistent HTTP (cont.)
4. HTTP server closes TCP
5. HTTP client receives response
time connection.
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

24. Non-Persistent HTTP: Response time
Definition of RTT: time to send a small
packet to travel from client to server
and back.
Response time: initiate TCP
 one RTT to initiate TCP connection connection
RTT
 one RTT for HTTP request and first
request
few bytes of HTTP response to return file
 file transmission time time to
RTT
transmit
total = 2RTT+transmit time file
file
received
time time
24

25. Persistent HTTP
Persistent HTTP
 server leaves connection open after sending response
 subsequent HTTP messages between same client/server sent
over open connection
Persistent without pipelining:
• client issues new request only when previous response has been
received
• one RTT for each referenced object
Persistent with pipelining:
 default in HTTP/1.1
 client sends requests as soon as it encounters a referenced object
 as little as one RTT for all the referenced objects

26. HTTP message: general format
two types of HTTP messages: request, response

27. HTTP request message
 HTTP request message:
 ASCII (human-readable format)
request line
(GET, POST, GET /somedir/page.html HTTP/1.1
HEAD commands) Host: www.someschool.edu
User-agent: Mozilla/4.0
header Connection: close
lines Accept-language:fr
Carriage return, (extra carriage return, line feed)
line feed
indicates end
of message

28. HTTP response message
status line
(protocol
HTTP/1.1 200 OK
status code
Connection close
status phrase)
Date: Thu, 06 Aug 1998 12:00:15 GMT
Server: Apache/1.3.0 (Unix)
header Last-Modified: Mon, 22 Jun 1998 …...
lines Content-Length: 6821
Content-Type: text/html
data, e.g., data data data data data ...
requested
HTML file

29. Method types
HTTP/1.1
HTTP/1.0  GET, POST, HEAD
 GET  PUT
 POST  uploads file in entity body to
 HEAD path specified in URL field
 asks server to leave requested  DELETE
object out of response  deletes file specified in the URL
field

30. 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

31. Internet Directory Service:
DNS: Domain Name System
People: many identifiers: Domain Name System:
 SSN, name, passport #  distributed database implemented
in hierarchy of many name servers
 Internet hosts, routers:
 application-layer protocol host,
 IP address (32 bit) - used routers, name servers to
for addressing datagrams communicate to resolve names
(address/name translation)
 “name”, e.g.,
 note: core Internet function,
www.yahoo.com - used implemented as application-
by humans layer protocol
Q: map between IP addresses  complexity at network’s “edge”
and name ?

32. DNS
DNS services Why not centralize DNS?
 Hostname to IP address  single point of failure
translation  traffic volume
 Host aliasing
 distant centralized database
 Canonical and alias  maintenance
names
 Mail server aliasing
doesn’t scale!
 Load distribution
 Replicated Web servers:
set of IP addresses for
one canonical name

33. Distributed, Hierarchical Database
Root DNS Servers
com DNS servers org DNS servers edu DNS servers
pbs.org poly.edu umass.edu
yahoo.com amazon.com
DNS servers DNS serversDNS servers
DNS servers DNS servers
Client wants IP for www.amazon.com; 1st approx:
 Client queries a root server to find com DNS server
 Client queries com DNS server to get amazon.com DNS server
 Client queries amazon.com DNS server to get IP address for
www.amazon.com,
33

34. 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
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also Los Angeles)
d U Maryland College Park, MD k RIPE London (also Amsterdam,
g US DoD Vienna, VA Frankfurt)
h ARL Aberdeen, MD i Autonomica, Stockholm (plus 3
j Verisign, ( 11 locations) other locations)
m WIDE Tokyo
e NASA Mt View, CA
f Internet Software C. Palo Alto,
CA (and 17 other locations)
13 root name servers
worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
34

35. TLD and Authoritative Servers
 Top-level domain (TLD) servers: responsible for com, org,
net, edu, etc, and all top-level country domains uk, fr, ca, jp.
 Network solutions maintains servers for com TLD
 Educause for edu TLD
 Authoritative DNS servers: organization’s DNS servers,
providing authoritative hostname to IP mappings for
organization’s servers (e.g., Web and mail).
 Can be maintained by organization or service provider

36. Local Name Server
 Does not strictly belong to hierarchy
 Each ISP (residential ISP, company, university) has one.
 Also called “default name server”
 When a host makes a DNS query, query is sent to its local
DNS server
 Acts as a proxy, forwards query into hierarchy.

37. Example
root DNS server
Host at cis.poly.edu wants IP
address for gaia.cs.umass.edu
2
3
TLD DNS server
4
5
local DNS server
dns.poly.edu
7 6
1 8
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
37
gaia.cs.umass.edu

38. Recursive queries
root DNS server
recursive query:
 puts burden of name
resolution on contacted 2 3
name server
 heavy load? 7 6
iterated query: TLD DNS server
 contacted server replies with
name of server to contact local DNS server
 “I don’t know this name, but dns.poly.edu 5 4
ask this server”
1 8
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
gaia.cs.umass.edu

39. DNS: caching and updating records
 once (any) name server learns mapping, it caches mapping
 cache entries timeout (disappear) after some time
 TLD servers typically cached in local name servers
 Thus root name servers not often visited
 update/notify mechanisms under design by IETF
 RFC 2136
 http://www.ietf.org/html.charters/dnsind-charter.html

40. DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)
Type=A Type=CNAME
 name is hostname  name is alias name for some
 value is IP address “canonical” (the real) name
www.ibm.com is really
Type=NS servereast.backup2.ibm.com
 name is domain (e.g. foo.com)  value is canonical name
 value is hostname of
authoritative name server for this
domain Type=MX
 value is name of mailserver
associated with name

41. 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
41

42. DNS protocol, messages
Name, type fields
for a query
RRs in response
to query
records for
authoritative servers
additional “helpful”
info that may be used

43. Inserting records into DNS
 Example: just created startup “Network Utopia”
 Register name networkuptopia.com at a registrar (e.g., Network
Solutions)
 Need to provide registrar with names and IP addresses of your
authoritative name server (primary and secondary)
 Registrar inserts two RRs into the com TLD server:
 (networkutopia.com, dns1.networkutopia.com, NS)
 (dns1.networkutopia.com, 212.212.212.1, A)
 Put in authoritative server Type A record for www.networkuptopia.com
and Type MX record for networkutopia.com
 How do people get the IP address of your Web site?

44. For students to do
Cookies
Web Caching

45. User-server state: cookies
Example:
Many major Web sites use
 Susan access Internet always
cookies
from same PC
Four components:  She visits a specific e-
1) cookie header line of commerce site for first time
HTTP response message  When initial HTTP requests
arrives at site, site creates a
2) cookie header line in unique ID and creates an entry
HTTP request message in backend database for ID
 Set-cookie : 1678453 included
3) cookie file kept on
in http response message which
user’s host, managed by browser saves in special cookie-
user’s browser file and sends it with every
request.
4) back-end database at
Web site

46. Cookies: keeping “state” (cont.)
client server
Cookie file usual http request msg en
server da try i
tab n b
usual http response + creates ID as ac
e
ebay: 8734 k en
Set-cookie: 1678 1678 for user d
Cookie file
usual http request msg
amazon: 1678 cookie: 1678 cookie- ss
ebay: 8734 specific acce
usual http response msg action
ss
one week later:
ce
ac
Cookie file usual http request msg
cookie-
amazon: 1678
cookie: 1678
spectific
ebay: 8734 usual http response msg action

47. Cookies (continued)
What cookies can bring:
 authorization
aside
 shopping carts Cookies and privacy:
 cookies permit sites to learn a lot
 recommendations
about you
 user session state (Web e-  you may supply name and e-mail to
sites
mail)
How to keep “state”:
 Protocol endpoints: maintain state at sender/receiver over
multiple transactions
 cookies: http messages carry state

48. Web caches (proxy server)
Goal: satisfy client request without involving origin server
 user sets browser: Web
accesses via cache origin
server
 browser sends all HTTP
HT Proxy
requests to cache TP
req server que
st
HT ues re
client TP TP ons
e
 object in cache: cache res
pon
t HT
res
p
se TP
returns object HT
st
ue
eq
 else cache requests Pr on
se
TT p
H res
object from origin HTT
P
server, then returns client
object to client origin
server

49. More about Web caching
 Cache acts as both client and Why Web caching?
server  Reduce response time for
 Typically cache is installed client request.
by ISP (university, company,  Reduce traffic on an
residential ISP) institution’s access link.
 Internet dense with caches:
enables “poor” content
providers to effectively
deliver content (but so does
P2P file sharing)

50. Caching example
Assumptions
origin
 average object size = 100,000 bits servers
 avg. request rate from institution’s
public
browsers to origin servers = 15/sec Internet
 delay from institutional router to any
origin server and back to router = 2 sec
Consequences 1.5 Mbps
access link
 utilization on LAN = 15%
institutional
 utilization on access link = 100% network
10 Mbps LAN
 total delay = Internet delay + access
delay + LAN delay
 = 2 sec + minutes + milliseconds

51. Caching example (cont)
Possible solution origin
servers
 increase bandwidth of access
public
link to, say, 10 Mbps Internet
Consequences
 utilization on LAN = 15%
10 Mbps
 utilization on access link = 15% access link
 Total delay = Internet delay + institutional
network
access delay + LAN delay 10 Mbps LAN
 = 2 sec + msecs + msecs
 often a costly upgrade

52. Caching example (cont)
Install cache
 suppose hit rate is .4
origin
servers
Consequence
public
 40% requests will be satisfied
Internet
almost immediately
 60% requests satisfied by
origin server
1.5 Mbps
 utilization of access link
access link
reduced to 60%, resulting in institutional
negligible delays (say 10 network
10 Mbps LAN
msec)
 total avg delay = Internet
delay + access delay + LAN
delay = .6*(2.01) secs + . institutional
4*milliseconds < 1.4 secs cache

53. Conditional GET
cache server
HTTP request msg
 Goal: don’t send object if cache If-modified-since:
has up-to-date cached version <date>
object
not
 cache: specify date of cached modified
copy in HTTP request HTTP response
HTTP/1.0
 If-modified-since: <date> 304 Not Modified
 server: response contains no
object if cached copy is up-to- HTTP request msg
date: If-modified-since:
HTTP/1.0 304 Not Modified <date> object
modified
HTTP response
HTTP/1.0 200 OK
<data>

54. Electronic Mail
outgoing
Three major components: message queue
 user agents user user mailbox
 mail servers agent
 simple mail transfer protocol: SMTP mail
user
server
agent
SMTP
User Agent mail
 a.k.a. “mail reader” server user
SMTP agent
 composing, editing, reading mail
messages SMTP
user
 e.g., Eudora, Outlook, elm, Netscape mail
server agent
Messenger
 outgoing, incoming messages stored user
on server agent
user
agent

55. Electronic Mail: mail servers
user
Mail Servers agent
mail
 mailbox contains incoming user
server
agent
messages for user SMTP
mail
 message queue of outgoing (to
server user
be sent) mail messages SMTP agent
 SMTP protocol between mail
SMTP
servers to send email messages mail user
server agent
 client: sending mail server
 “server”: receiving mail user
agent
server user
agent

56. Electronic Mail: SMTP [RFC 2821]
 uses TCP to reliably transfer email message 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

57. Sample SMTP interaction
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM: <alice@crepes.fr>
S: 250 alice@crepes.fr... Sender ok
C: RCPT TO: <bob@hamburger.edu>
S: 250 bob@hamburger.edu ... Recipient ok
C: DATA
S: 354 Enter mail, end with "." on a line by itself
C: Do you like ketchup?
C: How about pickles?
C: .
S: 250 Message accepted for delivery
C: QUIT
S: 221 hamburger.edu closing connection

58. SMTP: final words
 SMTP uses persistent
connections
 SMTP requires message
(header & body) to be in 7-bit
ASCII
 SMTP server uses
CRLF.CRLF to determine end
of message

59. Mail message format
SMTP: protocol for exchanging
email msgs header
blank
RFC 822: standard for text line
message format:
 header lines, e.g.,
body
 To:
 From:
 Subject:
 different from SMTP commands!
 body
 the “message”, ASCII characters
only

60. Message format: multimedia extensions
 MIME(multi purpose internet mail extensions): multimedia
mail extension, RFC 2045, 2056
 additional lines in msg header declare MIME content type
From: alice@crepes.fr
MIME version To: bob@hamburger.edu
Subject: Picture of yummy crepe.
method used MIME-Version: 1.0
to encode data Content-Transfer-Encoding: base64
Content-Type: image/jpeg
multimedia data
type, subtype, base64 encoded data .....
parameter declaration .........................
......base64 encoded data
encoded data

61. Scenario: Alice sends message to Bob
1) Alice uses UA to compose message 4) SMTP client sends Alice’s
and “to” bob@someschool.edu message over the TCP
2) Alice’s UA sends message to her connection
mail server; message placed in 5) Bob’s mail server places the
message queue message in Bob’s mailbox
3) Client side of SMTP opens TCP 6) Bob invokes his user agent to
connection with Bob’s mail server read message
1 mail
mail
server user
user server
2 agent
agent 3 6
4 5

62. Mail access protocols
SMTP SMTP access
user
user protocol
agent
agent
sender’s mail receiver’s mail
server 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.

63. POP3 protocol
S: +OK POP3 server ready
authorization phase C: user bob
 client commands: S: +OK
C: pass hungry
 user: declare username
S: +OK user successfully logged on
 pass: password
C: list
 server responses S: 1 498
 +OK S: 2 912
S: .
 -ERR
C: retr 1
transaction phase, client: S: <message 1 contents>
S: .
 list: list message numbers C: dele 1
 retr: retrieve message by number C: retr 2
S: <message 1 contents>
 dele: delete S: .
 quit C: dele 2
C: quit
S: +OK POP3 server signing off

64. POP3 (more) and IMAP
More about POP3 IMAP
 Previous example uses  Keep all messages in one
“download and delete” mode. place: the server
 Bob cannot re-read e-mail if  Allows user to organize
he changes client messages in folders
 “Download-and-keep”: copies  IMAP keeps user state across
of messages on different sessions:
clients  names of folders and
 POP3 is stateless across mappings between
sessions message IDs and folder
name

65. FTP: the file transfer protocol
2: Application
Layer
FTP file transfer
FTP FTP
user client server
interface
user
at host local file remote file
system system
 transfer file to/from remote host
 client/server model
 client: side that initiates transfer (either to/from remote)
 server: remote host
 ftp: RFC 959
 ftp server: port 21
65

66. FTP: separate control, data connections
2: Application
Layer
TCP control connection
 FTP client contacts FTP server at port 21
port 21, specifying TCP as
transport protocol
TCP data connection
 Client obtains authorization over FTP port 20 FTP
control connection client server
 Client browses remote directory Ì Server opens another TCP
by sending commands over
data connection to transfer
control connection.
another file.
 When server receives file transfer Ì Control connection: “out of
command, server opens 2nd TCP band”
connection (for file) to client Ì FTP server maintains “state”:
 After transferring one file, server current directory, earlier
closes data connection. authentication
66

67. FTP commands, responses
2: Application
Layer
Sample commands: Sample return codes
 sent as ASCII text over control  status code and phrase (as in
channel HTTP)
 USER username  331 Username OK,
 PASS password password required
 125 data connection
 LIST return list of file in
already open; transfer
current directory
starting
 RETR filename retrieves  425 Can’t open data
(gets) file connection
 STOR filename stores (puts)  452 Error writing file
file onto remote host
67

68. P2P file sharing
2: Application
Layer
Example
 Alice chooses one of the
peers, Bob.
 Alice runs P2P client
 File is copied from Bob’s
application on her
notebook computer PC to Alice’s notebook:
HTTP
 Intermittently connects to
 While Alice downloads,
Internet; gets new IP
address for each other users uploading from
connection Alice.
 Alice’s peer is both a Web
 Asks for “Hey Jude”
client and a transient Web
 Application displays other
server.
peers that have copy of
Hey Jude. All peers are servers = highly
scalable!
68

69. P2P: centralized directory
Bob
original “Napster” design centralized
directory server
1) when peer connects, it 1
informs central server: peers
1
 IP address
 content
1 3
2) Alice queries for “Hey 2 1
Jude”
3) Alice requests file from Bob
Alice
2: Application Layer 69

70. P2P: problems with centralized directory
2: Application
Layer
 Single point of failure file transfer is
 Performance bottleneck decentralized, but
locating content is highly
 Copyright infringement
centralized
70

71. Query flooding: Gnutella
2: Application
Layer
 fully distributed overlay network: graph
 no central server  edge between peer X and Y
 public domain protocol if there’s a TCP connection
 many Gnutella clients  all active peers and edges
implementing protocol is overlay net
 Edge is not a physical link
 Given peer will typically
be connected with < 10
overlay neighbors
71

72. Gnutella: protocol
File transfer:
72
HTTP
Query message
sent over existing TCP
connections Query
QueryHit
peers forward
Query message y Qu
u er ery
t
Q
y Hi
QueryHit
u er
sent over Q
reverse Query
path
QueryHit
Scalability: Qu
er
limited scope y
flooding
2: Application Layer

73. 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.
o r d in a r y p e e r
g r o u p - le a d e r p e e r
n e ig h o r in g r e la tio n s h ip s
in o v e r la y n e tw o r k
2: Application Layer 73

74. KaZaA: Querying
74
 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
2: Application Layer

75. KaZaA tricks
75
 Limitations on simultaneous uploads
 Request queuing
 Incentive priorities
 Parallel downloading
2: Application Layer

76. Multimedia
 Introduction to Audio
 Audio Compression
 Streaming Audio
 Internet Radio
 Voice over IP
 Introduction to Video
 Video Compression
 Video on Demand
 The MBone – The Multicast Backbone