1. Chapter 2
Application Layer
A note on the use of these ppt slides:
We’re making these slides freely available to all (faculty, students, readers).
They’re in PowerPoint form so you can add, modify, and delete slides
(including this one) and slide content to suit your needs. They obviously Computer Networking:
represent a lot of work on our part. In return for use, we only ask the A Top Down Approach,
following:
If you use these slides (e.g., in a class) in substantially unaltered form, 5th edition.
that you mention their source (after all, we’d like people to use our book!) Jim Kurose, Keith Ross
Addison-Wesley, April
If you post any slides in substantially unaltered form on a www site, that
you note that they are adapted from (or perhaps identical to) our slides, and
note our copyright of this material. 2009.
Thanks and enjoy! JFK/KWR
All material copyright 1996-2009
J.F Kurose and K.W. Ross, All Rights Reserved
2: Application Layer 1
2. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
2.2 Web and HTTP with TCP
2.3 FTP 2.8 Socket programming
2.4 Electronic Mail with UDP
SMTP, POP3, IMAP
2.5 DNS
2: Application Layer 2
3. Chapter 2: Application Layer
Our goals: learn about protocols
Conceptual(khái by examining popular
niệm),implementation( application-level
sự thi hành) aspects protocols
(khía cạnh)ofnetwork HTTP
application protocols FTP
transport-layer SMTP / POP3 / IMAP
servicemodels(kiểu) DNS
client-server programming network
paradigm(hệ biến applications
hoá) socket API
peer-to-peer
paradigm
2: Application Layer 3
4. Some network apps
e-mail voice over IP
web real-time video
instant messaging conferencing
remote login grid computing
P2P file sharing
multi-user network
games
streaming stored video
clips
2: Application Layer 4
5. Creating a network app application
transport
network
data link
write programs that physical
run on (different) end
systems
communicate over network
e.g., web server software
communicates with
browser software application
transport
No need to write software network
data link
for network-core devices
application
physical
transport
network
Network-core devices do data link
physical
not run user applications
applications on end systems
allows for rapid app
development, propagation
2: Application Layer 5
6. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
2.2 Web and HTTP with TCP
2.3 FTP 2.8 Socket programming
2.4 Electronic Mail with UDP
SMTP, POP3, IMAP 2.9 Building a Web
2.5 DNS server
2: Application Layer 6
8. Client-server architecture
server:
always-on host
permanent IP address
server farms for
scaling
clients:
client/server
communicate with server
may be intermittently
(không liên tục)connected
may have dynamic IP
addresses
do not communicate
directly with each other
2: Application Layer 8
9. Pure P2P architecture
no always-on server
Arbitrary(bất kỳ) end
systems directly peer-peer
communicate
peers are intermittently
connected and change IP
addresses
Highly scalable(có thể thay
đổi được ) but difficult
to manage
2: Application Layer 9
10. Hybrid of client-server and P2P
Skype
voice-over-IP P2P application
centralized server: finding address of remote
party(thuê bao):
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(bạn thân)
2: Application Layer 10
11. Processes communicating
Process: program running Client process: process
within a host. that initiates
within same host, two communication
processes communicate Server process: process
using inter-process that waits to be
communication (defined contacted
by OS).
processes in different Note: applications with
hosts communicate by P2P architectures have
exchanging messages client processes &
server processes
2: Application Layer 11
12. Sockets
process sends/receives host or host or
messages to/from its server server
socket controlled by
socket analogous(tương process
app developer
process
tự) to door socket socket
sending process shoves TCP with
TCP with
(đẩy)message out door buffers, Internet buffers,
sending process relies on variables variables
transport infrastructure
on other side of door which controlled
brings message to socket by OS
at receiving process
API: (1) choice of transport protocol; (2) ability to fix
a few parameters (lots more on this later)
2: Application Layer 12
13. Addressing processes
to receive messages,
process must have
identifier
host device has unique
32-bit IP address
Q: does IP address of
host suffice for
identifying the process?
2: Application Layer 13
14. Addressing processes
to receive messages, identifier includes both
process must have IP address and port
identifier numbers (chữ số để xác
host device has unique định port)associated
32-bit IP address with process on host.
Q: does IP address of Example port numbers:
host on which process HTTP server: 80
runs suffice for Mail server: 25
identifying the to send HTTP message
process? to gaia.cs.umass.edu web
A: No, many server:
processes can be IP address: 128.119.245.12
running on same host Port number: 80
more shortly…
2: Application Layer 14
15. App-layer protocol defines
Types of messages Public-domain protocols:
exchanged, defined in RFCs
e.g., request, response
allows for
Message syntax(cú pháp): interoperability(sự
what fields in messages &
tương kết)
how fields are
delineated(mô tả,) e.g., HTTP, SMTP
Message semantics Proprietary(cá nhân)
(semantics) protocols:
meaning of information in e.g., Skype
fields
Rules for when and how
processes send & respond
to messages 2: Application Layer 15
16. What transport service does an app need?
Data loss Throughput
some apps (e.g., audio) can some apps (e.g.,
tolerate some loss multimedia) require
other apps (e.g., file minimum amount of
transfer, telnet) require throughput to be
100% reliable data “effective”
transfer other apps (“elastic apps”)
Timing make use of whatever
some apps (e.g., throughput they get
Internet telephony,
interactive games) Security
require low delay to be Encryption, data
“effective” integrity(tính toàn vẹn), …
2: Application Layer 16
17. Transport service requirements of common apps
Application Data loss Throughput 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
2: Application Layer 17
18. 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 (làm tràn) receiver congestion control, timing,
throughput guarantee, or
congestion (tắc
security
nghẽn)control: throttle(tiết
lưu) sender when network
overloaded Q: why bother? Why is
does not provide: timing, there a UDP?
minimum throughput
guarantees, security 2: Application Layer 18
19. 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 HTTP (eg Youtube), TCP or UDP
RTP [RFC 1889]
Internet telephony SIP, RTP, proprietary
(e.g., Skype) typically UDP
2: Application Layer 19
20. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
app architectures with TCP
app requirements
2.8 Socket programming
2.2 Web and HTTP with UDP
2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2: Application Layer 20
21. 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
2: Application Layer 21
22. HTTP overview
HTTP: hypertext(siêu
văn bản) transfer HT
TP
r
equ
protocol PC running HT
TP
est
Explorer res
Web’s application layer pon
se
protocol
client/server model st
ue
client: browser that eq
TPr on
se Server
requests, receives, HT res
p running
TP Apache Web
“displays” Web objects HT
server
server: Web server
sends objects in Mac running
response to requests Navigator
2: Application Layer 22
23. HTTP overview (continued)
Uses TCP: HTTP is “stateless”
client initiates TCP server maintains no
connection (creates socket) information about
to server, port 80 past client requests
server accepts TCP
connection from client aside
Protocols that maintain “state”
HTTP messages (application- are complex!
layer protocol messages) past history (state) must be
exchanged between browser maintained
(HTTP client) and Web
if server/client crashes(bị
server (HTTP server)
hỏng), their views of “state”
TCP connection closed may be inconsistent(không
vững ), must be
reconciled(điều hòa)
2: Application Layer 23
24. HTTP connections
Nonpersistent HTTP Persistent HTTP
At most one object is Multiple objects can
sent over a TCP be sent over single
connection. TCP connection
between client and
server.
2: Application Layer 24
25. Nonpersistent HTTP
(contains text,
Suppose user enters URL references to 10
www.someSchool.edu/someDepartment/home.index jpeg images)
1a. HTTP client initiates TCP
connection to HTTP server
(process) at
1b. HTTP server at host
www.someSchool.edu waiting
www.someSchool.edu on port 80
for TCP connection at port 80.
“accepts” connection,
notifying client
2. HTTP client sends HTTP
request message (containing
URL) into TCP connection 3. HTTP server receives request
socket. Message indicates message, forms response
that client wants object message containing requested
someDepartment/home.index object, and sends message
into its socket
time
2: Application Layer 25
26. Nonpersistent HTTP (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
time 6. Steps 1-5 repeated for each
of 10 jpeg objects
2: Application Layer 26
27. Non-Persistent HTTP: Response time
Definition of RTT: time for
a small packet to travel
from client to server
and back. initiate TCP
connection
Response time: RTT
one RTT to initiate TCP request
file
connection time to
RTT
transmit
one RTT for HTTP file
request and first few file
received
bytes of HTTP response
to return time time
file transmission time
total = 2RTT+transmit time
2: Application Layer 27
28. Persistent HTTP
Nonpersistent HTTP issues: Persistent HTTP
requires 2 RTTs per object server leaves connection
OS overhead for each TCP open after sending
connection response
browsers often open parallel subsequent HTTP messages
TCP connections to fetch between same
referenced objects client/server sent over
open connection
client sends requests as
soon as it encounters a
referenced object
as little as one RTT for all
the referenced objects
2: Application Layer 28
29. HTTP request message
two types of HTTP messages: request, response
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
2: Application Layer 29
31. Uploading form input
Post method:
Web page often
includes form input URL method:
Input is uploaded to Uses GET method
server in entity body Input is uploaded in
URL field of request
line:
www.somesite.com/animalsearch?monkeys&banana
2: Application Layer 31
32. Method types
HTTP/1.0 HTTP/1.1
GET GET, POST, HEAD
POST PUT
HEAD uploads file in entity
body to path specified
asks server to leave
in URL field
requested object out of
response DELETE
deletes file specified in
the URL field
2: Application Layer 32
33. HTTP response message
status line
(protocol
status code HTTP/1.1 200 OK
status phrase) Connection close
Date: Thu, 06 Aug 1998 12:00:15 GMT
header Server: Apache/1.3.0 (Unix)
lines Last-Modified: Mon, 22 Jun 1998 …...
Content-Length: 6821
Content-Type: text/html
data, e.g., data data data data data ...
requested
HTML file
2: Application Layer 33
34. 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
2: Application Layer 34
35. Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
telnet cis.poly.edu 80 Opens TCP connection to port 80
(default HTTP server port) at cis.poly.edu.
Anything typed in sent
to port 80 at cis.poly.edu
2. Type in a GET HTTP request:
GET /~ross/ HTTP/1.1 By typing this in (hit carriage
Host: cis.poly.edu return twice), you send
this minimal (but complete)
GET request to HTTP server
3
2: Application Layer 35
36. User-server state: cookies
Example:
Many major Web sites
use cookies Susan always access
Four components: Internet always from PC
1) cookie header line of visits specific e-
HTTP response message commerce site for first
2) cookie header line in time
HTTP request message
3) cookie file kept on when initial HTTP
user’s host, managed by requests arrives at site,
user’s browser
site creates:
4) back-end database at
Web site unique ID
entry in backend
database for ID
2: Application Layer 36
38. Cookies (continued)
aside
What cookies can bring: Cookies and privacy:
authorization cookies permit sites to
shopping carts learn a lot about you
recommendations you may supply name
and e-mail to sites
user session state
(Web e-mail)
How to keep “state”:
protocol endpoints: maintain state
at sender/receiver over multiple
transactions
cookies: http messages carry state
2: Application Layer 38
39. Web caches (proxy server)
Goal: satisfy client request without involving origin server
user sets browser: origin
server
Web accesses via
t
es
cache Proxy
qu
re
browser sends all server
P
TT
clientHTTP on s
e
H
HTTP requests to re
HTTPsrequest esp
pon Pr
se HTT
cache
t
es
se
qu
on
object in cache: cache
re
sp
TP
returns object
re
HT
TP
else cache requests HT
object from origin client
origin
server, then returns server
object to client
2: Application Layer 39
40. More about Web caching
cache acts as both Why Web caching?
client and server reduce response time
typically cache is for client request
installed by ISP reduce traffic on an
(university, company, institution’s access
residential ISP) link.
Internet dense with
caches: enables “poor”
content providers to
effectively deliver
content (but so does
P2P file sharing)
2: Application Layer 40
41. Caching example
origin
Assumptions servers
average object size = 100,000
public
bits Internet
avg. request rate from
institution’s browsers to
origin servers = 15/sec
1.5 Mbps
delay from institutional router access link
to any origin server and back
institutional
to router = 2 sec network
10 Mbps LAN
Consequences
utilization on LAN = 15%
utilization on access link = 100%
institutional
total delay = Internet delay + cache
access delay + LAN delay
= 2 sec + minutes + milliseconds
2: Application Layer 41
42. Caching example (cont)
origin
possible solution servers
increase bandwidth of access
public
link to, say, 10 Mbps Internet
consequence
utilization on LAN = 15%
utilization on access link = 15% 10 Mbps
Total delay = Internet delay + access link
access delay + LAN delay institutional
= 2 sec + msecs + msecs network
10 Mbps LAN
often a costly upgrade
institutional
cache
2: Application Layer 42
43. Caching example (cont)
origin
possible solution: install servers
cache public
suppose hit rate is 0.4 Internet
consequence
40% requests will be
satisfied almost immediately
1.5 Mbps
60% requests satisfied by access link
origin server
utilization of access link institutional
reduced to 60%, resulting in network
10 Mbps LAN
negligible delays (say 10
msec)
total avg delay = Internet
delay + access delay + LAN institutional
delay = .6*(2.01) secs + .
cache
4*milliseconds < 1.4 secs
2: Application Layer 43
44. Conditional GET
Goal: don’t send object if cache server
cache has up-to-date cached HTTP request msg
version If-modified-since:
object
cache: specify date of <date>
not
cached copy in HTTP request modified
HTTP response
If-modified-since: HTTP/1.0
<date> 304 Not Modified
server: response contains no
object if cached copy is up-
HTTP request msg
to-date: If-modified-since:
HTTP/1.0 304 Not <date> object
Modified modified
HTTP response
HTTP/1.0 200 OK
<data>
2: Application Layer 44
45. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
2.2 Web and HTTP with TCP
2.3 FTP 2.8 Socket programming
2.4 Electronic Mail with UDP
SMTP, POP3, IMAP 2.9 Building a Web
2.5 DNS server
2: Application Layer 45
46. FTP: the file transfer protocol
FTP file transfer
FTP FTP
user client server
interface
user
at host remote file
local 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
2: Application Layer 46
47. FTP: separate control, data connections
TCP control connection
FTP client contacts FTP server port 21
at port 21, TCP is transport
protocol TCP data connection
client authorized over control FTP port 20 FTP
connection client server
client browses remote
server opens another TCP
directory by sending commands
data connection to transfer
over control connection.
another file.
when server receives file
control connection: “out of
transfer command, server
band”
opens 2nd TCP connection (for
file) to client FTP server maintains “state”:
current directory, earlier
after transferring one file,
authentication
server closes data connection.
2: Application Layer 47
48. FTP commands, responses
Sample commands: Sample return codes
sent as ASCII text over status code and phrase (as
control channel in HTTP)
USER username 331 Username OK,
PASS password password required
LIST return list of file in 125 data connection
current directory already open;
transfer starting
RETR filename retrieves
425 Can’t open data
(gets) file connection
STOR filename stores 452 Error writing
(puts) file onto remote file
host
2: Application Layer 48
49. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
2.2 Web and HTTP with TCP
2.3 FTP 2.8 Socket programming
2.4 Electronic Mail with UDP
SMTP, POP3, IMAP
2.5 DNS
2: Application Layer 49
50. Electronic Mail outgoing
message queue
user mailbox
user
Three major components: agent
user agents mail
user
server
mail servers agent
simple mail transfer SMTP mail
protocol: SMTP server user
SMTP agent
User Agent
a.k.a. “mail reader” SMTP
mail user
composing, editing, reading server agent
mail messages
e.g., Eudora, Outlook, elm, user
Mozilla Thunderbird agent
user
outgoing, incoming messages agent
stored on server
2: Application Layer 50
51. Electronic Mail: mail servers
user
Mail Servers agent
mailbox contains incoming mail
user
messages for user server
agent
message queue of outgoing
SMTP
(to be sent) mail messages mail
server user
SMTP protocol between mail
servers to send email SMTP agent
messages SMTP
client: sending mail mail user
agent
server server
“server”: receiving mail
user
server agent
user
agent
2: Application Layer 51
52. 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
2: Application Layer 52
53. Scenario: Alice sends message to Bob
1) Alice uses UA to compose 4) SMTP client sends Alice’s
message and “to” message over the TCP
bob@someschool.edu connection
2) Alice’s UA sends message 5) Bob’s mail server places the
to her mail server; message message in Bob’s mailbox
placed in message queue 6) Bob invokes his user agent
3) Client side of SMTP opens to read message
TCP connection with Bob’s
mail server
1 mail
mail
server user
user server
2 agent
agent 3 6
4 5
2: Application Layer 53
54. 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
2: Application Layer 54
55. Try SMTP interaction for 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)
2: Application Layer 55
56. SMTP: final words
SMTP uses persistent Comparison with HTTP:
connections
HTTP: pull
SMTP requires message
(header & body) to be in 7- SMTP: push
bit ASCII
both have ASCII
SMTP server uses command/response
CRLF.CRLF to determine interaction, status codes
end of message
HTTP: each object
encapsulated in its own
response msg
SMTP: multiple objects
sent in multipart msg
2: Application Layer 56
57. Mail message format
SMTP: protocol for
exchanging email msgs header
blank
RFC 822: standard for text
line
message format:
header lines, e.g.,
To: body
From:
Subject:
different from SMTP
commands!
body
the “message”, ASCII
characters only
2: Application Layer 57
58. Mail access protocols
SMTP SMTP access user
user
agent protocol 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: gmail, Hotmail, Yahoo! Mail, etc.
2: Application Layer 58
59. POP3 protocol S: +OK POP3 server ready
C: user bob
authorization phase S: +OK
C: pass hungry
client commands: S: +OK user successfully logged on
user: declare username
C: list
pass: password S: 1 498
server responses S: 2 912
+OK
S: .
C: retr 1
-ERR S: <message 1 contents>
transaction phase, client: S: .
C: dele 1
list: list message numbers C: retr 2
retr: retrieve message by S: <message 1 contents>
number S: .
C: dele 2
dele: delete C: quit
quit S: +OK POP3 server signing off
2: Application Layer 59
60. POP3 (more) and IMAP
More about POP3 IMAP
Previous example uses Keep all messages in
“download and delete” one place: the server
mode. Allows user to
Bob cannot re-read e- organize messages in
mail if he changes folders
client IMAP keeps user state
“Download-and-keep”: across sessions:
copies of messages on names of folders and
different clients mappings between
message IDs and folder
POP3 is stateless
name
across sessions
2: Application Layer 60
61. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
2.2 Web and HTTP with TCP
2.3 FTP 2.8 Socket programming
2.4 Electronic Mail with UDP
SMTP, POP3, IMAP 2.9 Building a Web
2.5 DNS server
2: Application Layer 61
62. DNS: Domain Name System
People: many identifiers: Domain Name System:
SSN, name, passport # distributed database
implemented in hierarchy of
Internet hosts, routers:
many name servers
IP address (32 bit) -
application-layer protocol
used for addressing
host, routers, name servers to
datagrams
communicate to resolve names
“name”, e.g., (address/name translation)
ww.yahoo.com - used by note: core Internet
humans
function, implemented as
Q: map between IP application-layer protocol
addresses and name ? complexity at network’s
“edge”
2: Application Layer 62
63. DNS
DNS services Why not centralize DNS?
hostname to IP single point of failure
address translation traffic volume
host aliasing distant centralized
Canonical, alias names database
mail server aliasing maintenance
load distribution
replicated Web servers: doesn’t scale!
set of IP addresses for
one canonical name
2: Application Layer 63
64. 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
2: Application Layer 64
65. 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 LA)
d U Maryland College Park, MD k RIPE London (also 16 other locations)
g US DoD Vienna, VA
h ARL Aberdeen, MD i Autonomica, Stockholm (plus
j Verisign, ( 21 locations) 28 other locations)
e NASA Mt View, CA m WIDE Tokyo (also Seoul,
f Internet Software C. Palo Alto, Paris, SF)
CA (and 36 other locations)
13 root name
servers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
2: Application Layer 65
66. 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, mail).
can be maintained by organization or service
provider
2: Application Layer 66
67. Local Name Server
does not strictly belong to hierarchy
each ISP (residential ISP, company,
university) has one.
also called “default name server”
when host makes DNS query, query is sent
to its local DNS server
acts as proxy, forwards query into hierarchy
2: Application Layer 67
68. DNS name root DNS server
resolution example
2
Host at cis.poly.edu 3
TLD DNS server
wants IP address for 4
gaia.cs.umass.edu 5
iterated query: local DNS server
dns.poly.edu
contacted server 7 6
replies with name of 1 8
server to contact
authoritative DNS server
“I don’t know this dns.cs.umass.edu
name, but ask this requesting host
server” cis.poly.edu
gaia.cs.umass.edu
2: Application Layer 68
69. DNS name
resolution example root DNS server
recursive query: 2 3
puts burden of name 6
7
resolution on
TLD DNS server
contacted name
server
heavy load? local DNS server
dns.poly.edu 5 4
1 8
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
gaia.cs.umass.edu
2: Application Layer 69
70. 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
2: Application Layer 70
71. 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.
value is canonical name
foo.com)
value is hostname of Type=MX
authoritative name value is name of mailserver
server for this domain
associated with name
2: Application Layer 71
72. 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
2: Application Layer 72
73. 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
2: Application Layer 73
74. Inserting records into DNS
example: new startup “Network Utopia”
register name networkuptopia.com at DNS registrar
(e.g., Network Solutions)
provide names, IP addresses of authoritative name server
(primary and secondary)
registrar inserts two RRs into com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A)
create authoritative server Type A record for
www.networkuptopia.com; Type MX record for
networkutopia.com
How do people get IP address of your Web site?
2: Application Layer 74
75. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
app architectures with TCP
app requirements
2.8 Socket programming
2.2 Web and HTTP with UDP
2.4 Electronic Mail
SMTP, POP3, IMAP
2.5 DNS
2: Application Layer 75
76. 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 76
77. File Distribution: Server-Client vs P2P
Question : How much time to distribute file
from one server to N peers?
us: server upload
Server
bandwidth
ui: peer i upload
u1 d1 u2 bandwidth
us d2
di: peer i download
File, size F bandwidth
dN
Network (with
uN abundant bandwidth)
2: Application Layer 77
78. File distribution time: server-client
Server
server sequentially F u1 d1 u2
sends N copies: us d2
NF/us time dN Network (with
abundant bandwidth)
client i takes F/di uN
time to download
Time to distribute F
to N clients using = max { NF/us, F/min(di) }
= dcs
i
client/server approach
increases linearly in N
(for large N) 2: Application Layer 78
79. File distribution time: P2P
Server
server must send one F u1 d1 u2
copy: F/us time us d2
client i takes F/di time dN Network (with
to download abundant bandwidth)
uN
NF bits must be
downloaded (aggregate)
fastest possible upload rate: us + Σui
dP2P = max { F/us, F/min(di) , NF/(us + Σui) }
i
2: Application Layer 79
80. Server-client vs. P2P: example
Client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
3.5
P2P
Minimum Distribution Time
3
Client-Server
2.5
2
1.5
1
0.5
0
0 5 10 15 20 25 30 35
N
2: Application Layer 80
81. File distribution: BitTorrent
P2P file distribution
tracker: tracks peers torrent: group of
participating in torrent peers exchanging
chunks of a file
obtain list
of peers
trading
chunks
peer
2: Application Layer 81
82. 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 82
83. BitTorrent (2) Sending Chunks: tit-for-tat
Pulling Chunks Alice sends chunks to four
neighbors currently
at any given time, sending her chunks at the
different peers have highest rate
different subsets of
re-evaluate top 4 every
file chunks
10 secs
periodically, a peer
(Alice) asks each every 30 secs: randomly
neighbor for list of select another peer,
chunks that they have. starts sending chunks
newly chosen peer may
Alice sends requests
for her missing chunks join top 4
“optimistically unchoke”
rarest first
2: Application Layer 83
84. 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 84
85. 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
86. 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
87. 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
88. Circular DHT (1)
1
3
15
4
12
5
10
8
Each peer only aware of immediate successor
and predecessor.
“Overlay network”
89. Circle DHT (2)
O(N) messages 0001 Who’s resp
on avg to resolve for key 1110 ?
I am
query, when there 0011
are N peers
1111
1110
1110
0100
1110
1100
1110
1110 0101
Define closest 1110
as closest 1010
successor 1000
90. Circular DHT with Shortcuts
12
15
Who’s resp
for key 1110?
10
8
1
3
5
4
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
91. Peer Churn
1
•To handle peer churn, require
3 each peer to know the IP address
15
of its two successors.
• Each peer periodically pings its
4
two successors to see if they
12 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?
92. P2P Case study: Skype
Skype clients (SC)
inherently P2P: pairs
of users communicate.
proprietary Skype
application-layer login server Supernode
protocol (inferred via (SN)
reverse engineering)
hierarchical overlay
with SNs
Index maps usernames
to IP addresses;
distributed over SNs
2: Application Layer 92
93. 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 93
94. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
2.2 Web and HTTP with TCP
2.3 FTP 2.8 Socket programming
2.4 Electronic Mail with UDP
SMTP, POP3, IMAP
2.5 DNS
2: Application Layer 94
95. Socket programming
Goal: learn how to build client/server application that
communicate using sockets
Socket API socket
introduced in BSD4.1 UNIX,
a host-local,
1981
application-created,
explicitly created, used, OS-controlled interface
released by apps (a “door”) into which
client/server paradigm application process can
two types of transport both send and
service via socket API: receive messages to/from
unreliable datagram another application
process
reliable, byte stream-
oriented
2: Application Layer 95
96. 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
controlled by process application
application process
developer
developer socket socket
TCP with TCP with controlled by
controlled by
buffers, operating
operating buffers, internet system
system variables variables
host or host or
server server
2: Application Layer 96
97. Socket programming with TCP
Client must contact server When contacted by client,
server process must first server TCP creates new
be running socket for server process to
server must have created communicate with client
socket (door) that allows server to talk with
welcomes client’s contact multiple clients
source port numbers
Client contacts server by:
used to distinguish
creating client-local TCP clients (more in Chap 3)
socket
specifying IP address, port application viewpoint
number of server process
TCP provides reliable, in-order
When client creates transfer of bytes (“pipe”)
socket: client TCP between client and server
establishes connection to
server TCP
2: Application Layer 97
98. Client/server socket interaction: TCP
Server (running on hostid) Client
create socket,
port=x, for
incoming request:
welcomeSocket =
ServerSocket()
TCP create socket,
wait for incoming
connection request connection setup connect to hostid, port=x
connectionSocket = clientSocket =
welcomeSocket.accept() Socket()
send request using
read request from clientSocket
connectionSocket
write reply to
connectionSocket read reply from
clientSocket
close
connectionSocket close
clientSocket
2: Application Layer 98
99. Stream jargon
keyboard monitor
A stream is a sequence of
characters that flow into
or out of a process.
inFromUser
input
stream
An input stream is Client
attached to some input
Process
process
source for the process,
e.g., keyboard or socket.
An output stream is
attached to an output
inFromServer
outToServer
output input
source, e.g., monitor or stream stream
socket.
client TCP
clientSocket
socket TCP
socket
to network from network
2: Application Layer 99
100. 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 100
101. 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 BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Create
client socket, Socket clientSocket = new Socket("hostname", 6789);
connect to server
Create DataOutputStream outToServer =
output stream new DataOutputStream(clientSocket.getOutputStream());
attached to socket
2: Application Layer 101
102. Example: Java client (TCP), cont.
Create BufferedReader inFromServer =
input stream new BufferedReader(new
attached to socket InputStreamReader(clientSocket.getInputStream()));
sentence = inFromUser.readLine();
Send line
to server outToServer.writeBytes(sentence + 'n');
Read line modifiedSentence = inFromServer.readLine();
from server
System.out.println("FROM SERVER: " + modifiedSentence);
clientSocket.close();
}
}
2: Application Layer 102
103. Example: Java server (TCP)
import java.io.*;
import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception
{
String clientSentence;
Create String capitalizedSentence;
welcoming socket
ServerSocket welcomeSocket = new ServerSocket(6789);
at port 6789
while(true) {
Wait, on welcoming
socket for contact Socket connectionSocket = welcomeSocket.accept();
by client
BufferedReader inFromClient =
Create input new BufferedReader(new
stream, attached InputStreamReader(connectionSocket.getInputStream()));
to socket
2: Application Layer 103
104. Example: Java server (TCP), cont
Create output
stream, attached DataOutputStream outToClient =
to socket new DataOutputStream(connectionSocket.getOutputStream());
Read in line
from socket clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + 'n';
Write out line
outToClient.writeBytes(capitalizedSentence);
to socket
}
}
} End of while loop,
loop back and wait for
another client connection
2: Application Layer 104
105. Chapter 2: Application layer
2.1 Principles of 2.6 P2P applications
network applications 2.7 Socket programming
2.2 Web and HTTP with TCP
2.3 FTP 2.8 Socket programming
2.4 Electronic Mail with UDP
SMTP, POP3, IMAP
2.5 DNS
2: Application Layer 105
106. Socket programming with UDP
UDP: no “connection” between
client and server
no handshaking
sender explicitly attaches application viewpoint
IP address and port of
destination to each packet UDP provides unreliable transfer
of groups of bytes (“datagrams”)
server must extract IP between client and server
address, port of sender
from received packet
UDP: transmitted data may be
received out of order, or
lost
2: Application Layer 106
107. Client/server socket interaction: UDP
Server (running on hostid) Client
create socket, create socket,
port= x. clientSocket =
serverSocket = DatagramSocket()
DatagramSocket()
Create datagram with server IP and
port=x; send datagram via
read datagram from clientSocket
serverSocket
write reply to
serverSocket
specifying read datagram from
client address, clientSocket
port number close
clientSocket
2: Application Layer 107
108. Example: Java client (UDP)
keyboard monitor
inFromUser
input
stream
Client
Process
Input: receives
process
packet (recall
Output: sends thatTCP received
packet (recall “byte stream”)
receivePacket
sendPacket
that TCP sent UDP
packet
UDP
packet
“byte stream”)
client UDP
clientSocket
socket UDP
socket
to network from network
2: Application Layer 108
109. Example: Java client (UDP)
import java.io.*;
import java.net.*;
class UDPClient {
public static void main(String args[]) throws Exception
{
Create
input stream BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
Create
client socket DatagramSocket clientSocket = new DatagramSocket();
Translate
InetAddress IPAddress = InetAddress.getByName("hostname");
hostname to IP
address using DNS byte[] sendData = new byte[1024];
byte[] receiveData = new byte[1024];
String sentence = inFromUser.readLine();
sendData = sentence.getBytes();
2: Application Layer 109
110. Example: Java client (UDP), cont.
Create datagram
with data-to-send, DatagramPacket sendPacket =
length, IP addr, port new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
Send datagram clientSocket.send(sendPacket);
to server
DatagramPacket receivePacket =
new DatagramPacket(receiveData, receiveData.length);
Read datagram
clientSocket.receive(receivePacket);
from server
String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
clientSocket.close();
}
}
2: Application Layer 110
111. Example: Java server (UDP)
import java.io.*;
import java.net.*;
class UDPServer {
public static void main(String args[]) throws Exception
Create {
datagram socket
DatagramSocket serverSocket = new DatagramSocket(9876);
at port 9876
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{
Create space for
DatagramPacket receivePacket =
received datagram
new DatagramPacket(receiveData, receiveData.length);
Receive serverSocket.receive(receivePacket);
datagram
2: Application Layer 111
112. Example: Java server (UDP), cont
String sentence = new String(receivePacket.getData());
Get IP addr
InetAddress IPAddress = receivePacket.getAddress();
port #, of
sender int port = receivePacket.getPort();
String capitalizedSentence = sentence.toUpperCase();
sendData = capitalizedSentence.getBytes();
Create datagram
DatagramPacket sendPacket =
to send to client new DatagramPacket(sendData, sendData.length, IPAddress,
port);
Write out
datagram serverSocket.send(sendPacket);
to socket }
}
} End of while loop,
loop back and wait for
another datagram
2: Application Layer 112
113. Chapter 2: Summary
our study of network apps now complete!
application architectures specific protocols:
HTTP
client-server
FTP
P2P
SMTP, POP, IMAP
hybrid
DNS
application service
P2P: BitTorrent, Skype
requirements:
reliability, bandwidth, socket programming
delay
Internet transport
service model
connection-oriented,
reliable: TCP
unreliable, datagrams: UDP
2: Application Layer 113
114. Chapter 2: Summary
Most importantly: learned about protocols
typical request/reply Important themes:
message exchange: control vs. data msgs
client requests info or in-band, out-of-band
service
server responds with centralized vs.
data, status code decentralized
message formats: stateless vs. stateful
headers: fields giving reliable vs. unreliable
info about data
msg transfer
data: info being
communicated “complexity at network
edge”
2: Application Layer 114