3rd edition chapter2

Chapter 2
Application Layer

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2: Application Layer 1

Chapter 2: Application layer
Ì 2.1 Principles of Ì 2.6 P2P file sharing
network applications Ì 2.7 Socket programming
Ì 2.2 Web and HTTP with TCP
Ì 2.3 FTP Ì 2.8 Socket programming
Ì 2.4 Electronic Mail with UDP
 SMTP, POP3, IMAP Ì 2.9 Building a Web
Ì 2.5 DNS server


Chapter 2: Application Layer
Our goals: Ì learn about protocols
Ì conceptual, by examining popular
implementation application-level
aspects of network protocols
application protocols  HTTP
 transport-layer  FTP
service models  SMTP / POP3 / IMAP
 client-server
 DNS
paradigm Ì programming network
 peer-to-peer applications
paradigm  socket API


Some network apps
Ì E-mail Ì Internet telephone
Ì Web Ì Real-time video
Ì Instant messaging conference
Ì Remote login Ì Massive parallel
Ì P2P file sharing computing
Ì
Ì Multi-user network
games Ì

Ì Streaming stored Ì
video clips


Creating a network app
Write programs that application
transport
 run on different end network
data link
systems and physical

 communicate over a
network.
 e.g., Web: Web server
software communicates
with browser software
little software written for
devices in network core application
application
transport
 network core devices do transport
network
network
data link
not run user application data link
physical
physical

code
 application on end systems
allows for rapid app
development, propagation

Ì 2.5 DNS server


Application architectures
Ì Client-server
Ì Peer-to-peer (P2P)
Ì Hybrid of client-server and P2P


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


Pure P2P architecture
Ì no always-on server
Ì arbitrary end systems
directly communicate
Ì peers are intermittently
connected and change IP
addresses
Ì example: Gnutella

Highly scalable

But difficult to manage

Hybrid of client-server and P2P
Napster
 File transfer P2P
 File search centralized:
• Peers register content at central server
• Peers query same central server to locate content
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


Processes communicating
Process: program running Client process: process
within a host. that initiates
Ì within same host, two communication
processes communicate Server process: process
using inter-process that waits to be
communication (defined contacted
by OS).
Ì processes in different Ì Note: applications with
hosts communicate by P2P architectures have
exchanging messages client processes &
server processes


Sockets
host or host or
Ì process sends/receives server server
messages to/from its
socket controlled by
app developer
Ì socket analogous to door process process

socket
 sending process shoves socket
message out door TCP with TCP with
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

Ì API: (1) choice of transport protocol; (2) ability to fix
a few parameters (lots more on this later)

Addressing processes
Ì For a process to Ì Identifier includes
receive messages, it both the IP address
must have an identifier and port numbers
Ì A host has a unique32- associated with the
bit IP address process on the host.
Ì Q: does the IP address Ì Example port numbers:
of the host on which  HTTP server: 80
the process runs  Mail server: 25
suffice for identifying Ì More on this later
the process?
Ì Answer: No, many
processes can be
running on same host

App-layer protocol defines
Ì Types of messages Public-domain protocols:
exchanged, e.g., request Ì defined in RFCs
& response messages Ì allows for
Ì Syntax of message interoperability
types: what fields in Ì e.g., HTTP, SMTP
messages & how fields
are delineated Proprietary protocols:
Ì e.g., KaZaA
Ì Semantics of the fields,
i.e., meaning of
information in fields
Ì Rules for when and how
processes send &
respond to messages 2: Application Layer 14

What transport service does an app need?
Data loss Bandwidth
Ì some apps (e.g., audio) can Ì some apps (e.g.,
tolerate some loss multimedia) require
Ì other apps (e.g., file minimum amount of
transfer, telnet) require bandwidth to be
100% reliable data
“effective”
transfer
Ì other apps (“elastic
Timing
apps”) make use of
Ì some apps (e.g.,
whatever bandwidth
Internet telephony,
they get
interactive games)
require low delay to be
“effective”

Transport service requirements of common apps

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


Internet transport protocols services

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

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


 app architectures with TCP
 app requirements Ì 2.8 Socket programming
Ì 2.2 Web and HTTP with UDP
Ì 2.4 Electronic Mail Ì 2.9 Building a Web
 SMTP, POP3, IMAP
server
Ì 2.5 DNS


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

host name path name


HTTP overview

HTTP: hypertext
transfer protocol HT
TP
r
equ
Ì Web’s application layer PC running HT e st
TP
protocol Explorer res
pon
se
Ì client/server model
 client: browser that
st
requests, receives, e que
Pr on
se Server
“displays” Web objects TT p
H
P re s running
T Apache Web
 server: Web server HT
server
sends objects in
response to requests
Mac running
Ì HTTP 1.0: RFC 1945 Navigator
Ì HTTP 1.1: RFC 2068


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


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


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

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


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

Persistent HTTP

Nonpersistent HTTP issues: Persistent without pipelining:
Ì requires 2 RTTs per object Ì client issues new request
Ì OS overhead for each TCP only when previous
connection response has been received
Ì browsers often open parallel Ì one RTT for each
TCP connections to fetch referenced object
referenced objects Persistent with pipelining:
Persistent HTTP Ì default in HTTP/1.1
Ì server leaves connection Ì client sends requests as
open after sending response soon as it encounters a
Ì subsequent HTTP messages referenced object
between same client/server Ì as little as one RTT for all
sent over open connection the referenced objects


HTTP request message

Ì two types of HTTP messages: request, response
Ì 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

HTTP request message: general format


Uploading form input
Post method:
Ì Web page often
includes form input URL method:
Ì Input is uploaded to Ì Uses GET method
server in entity body Ì Input is uploaded in
URL field of request
line:

www.somesite.com/animalsearch?monkeys&banana


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


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


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

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. Look at response message sent by HTTP server!


Let’s look at HTTP in action
Ì telnet example
Ì Ethereal example


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


Cookies: keeping “state” (cont.)

client server
Cookie file usual http request msg server n e
da try i
tab n b
usual http response + creates ID as ac
e ke
ebay: 8734 Set-cookie: 1678 1678 for user nd

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
usual http request msg
Cookie file cookie-
cookie: 1678
amazon: 1678 spectific
ebay: 8734 usual http response msg action


Cookies (continued)
aside
What cookies can bring: Cookies and privacy:
Ì authorization Ì cookies permit sites to
Ì shopping carts learn a lot about you
Ì you may supply name
Ì recommendations
and e-mail to sites
Ì user session state
Ì search engines use
(Web e-mail)
redirection & cookies
to learn yet more
Ì advertising companies
obtain info across
sites


Web caches (proxy server)
Goal: satisfy client request without involving origin server

Ì user sets browser: Web origin
accesses via cache server

Ì browser sends all HTTP Proxy
HT st
TP
requests to cache req server reque
HT ues TP e
object in cache: cache client TP t HT ons
e sp
 res
returns object pon TPr
se HT
st
 else cache requests ue
eq se
object from origin T Pr on
HT p
server, then returns P res
HTT
object to client
client
origin
server


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


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

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

institutional
cache


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


Conditional GET

Ì Goal: don’t send object if cache server
cache has up-to-date cached HTTP request msg
version If-modified-since:
object
Ì cache: specify date of <date>
not
cached copy in HTTP request modified
HTTP response
If-modified-since: HTTP/1.0
<date> 304 Not Modified
Ì 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>

Ì 2.5 DNS server


FTP: the file transfer protocol

FTP file transfer
FTP FTP
user client server
interface
user
at host local file remote file
system system

Ì transfer file to/from remote host
Ì client/server model
 client: side that initiates transfer (either to/from
remote)
 server: remote host
Ì ftp: RFC 959
Ì ftp server: port 21


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

FTP commands, responses

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


Ì 2.5 DNS server


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

Electronic Mail: mail servers
user
Mail Servers agent
Ì mailbox contains incoming mail
user
messages for user server
agent
Ì message queue of outgoing
SMTP
(to be sent) mail messages mail
server user
Ì 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


Electronic Mail: SMTP [RFC 2821]
Ì uses TCP to reliably transfer email message from client
to server, port 25
Ì direct transfer: sending server to receiving server
Ì three phases of transfer
 handshaking (greeting)
 transfer of messages
 closure
Ì command/response interaction
 commands: ASCII text
 response: status code and phrase

Ì messages must be in 7-bit ASCII


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


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


Try SMTP interaction for yourself:

Ì telnet servername 25
Ì see 220 reply from server
Ì enter HELO, MAIL FROM, RCPT TO, DATA, QUIT
commands
above lets you send email without using email client
(reader)


SMTP: final words
Ì SMTP uses persistent Comparison with HTTP:
connections
Ì HTTP: pull
Ì SMTP requires message
Ì SMTP: push
(header & body) to be in 7-
bit ASCII Ì both have ASCII
Ì SMTP server uses command/response
CRLF.CRLF to determine interaction, status codes
end of message
Ì HTTP: each object
encapsulated in its own
response msg
Ì SMTP: multiple objects
sent in multipart msg


Mail message format

SMTP: protocol for
exchanging email msgs header
blank
RFC 822: standard for text
line
message format:
Ì header lines, e.g.,
 To: body
 From:
 Subject:

different from SMTP
commands!
Ì body
 the “message”, ASCII
characters only


Message format: multimedia extensions
Ì MIME: multimedia mail extension, RFC 2045, 2056
Ì 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


Mail access protocols
SMTP SMTP access user
user
agent protocol agent

sender’s mail receiver’s mail
server server
Ì SMTP: delivery/storage to receiver’s server
Ì 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.


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

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

Ì 2.5 DNS server


DNS: Domain Name System

People: many identifiers: Domain Name System:
 SSN, name, passport # Ì distributed database
implemented in hierarchy of
Internet hosts, routers:
many name servers
 IP address (32 bit) -
Ì 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”


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


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:
Ì Client queries a root server to find com DNS
server
Ì Client queries com DNS server to get amazon.com
DNS server
Ì Client queries amazon.com DNS server to get IP
address for www.amazon.com

DNS: Root name servers
Ì contacted by local name server that can not resolve name
Ì 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
h ARL Aberdeen, MD i Frankfurt) Stockholm (plus 3
Autonomica,
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


TLD and Authoritative Servers
Ì 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

Ì 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


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


Example root DNS server

2
Ì Host at cis.poly.edu 3
TLD DNS server
wants IP address for 4
gaia.cs.umass.edu
5

local DNS server
dns.poly.edu
7 6
1 8

authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu

gaia.cs.umass.edu


Recursive queries root DNS server

recursive query:
2 3
Ì puts burden of name
resolution on 7 6
contacted name TLD DNS serve
server
Ì heavy load?
local DNS server
4
iterated query: dns.poly.edu 5

Ì contacted server 1 8
replies with name of
server to contact authoritative DNS server
Ì “I don’t know this dns.cs.umass.edu
requesting host
name, but ask this cis.poly.edu
server”
gaia.cs.umass.edu

DNS: caching and updating records
Ì 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
Ì update/notify mechanisms under design by IETF
 RFC 2136
 http://www.ietf.org/html.charters/dnsind-charter.html


DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)

Ì Type=A Ì Type=CNAME
 name is hostname  name is alias name for some
 value is IP address “canonical” (the real) name
www.ibm.com is really
Ì Type=NS
servereast.backup2.ibm.com
 name is domain (e.g.
 value is canonical name
foo.com)
 value is hostname of Ì Type=MX
authoritative name  value is name of mailserver
server for this domain
associated with name


DNS protocol, messages
DNS protocol : query and reply messages, both with
same message format

msg header
Ì identification: 16 bit #
for query, reply to query
uses same #
Ì flags:
 query or reply
 recursion desired
 recursion available
 reply is authoritative


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


Inserting records into DNS
Ì Example: just created startup “Network Utopia”
Ì 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)

Ì Put in authoritative server Type A record for
www.networkuptopia.com and Type MX record for
networkutopia.com
Ì How do people get the IP address of your Web site?


server
Ì 2.5 DNS


P2P file sharing
Ì Alice chooses one of

Example the peers, Bob.
Ì Alice runs P2P client Ì File is copied from

application on her Bob’s PC to Alice’s
notebook computer notebook: HTTP
Ì Intermittently Ì While Alice downloads,

connects to Internet; other users uploading
gets new IP address from Alice.
for each connection Ì Alice’s peer is both a
Ì Asks for “Hey Jude” Web client and a
transient Web server.
Ì Application displays
other peers that have All peers are servers =
copy of Hey Jude. highly scalable!

P2P: centralized directory
Bob
original “Napster” design centralized
directory server
1) when peer connects, it 1
informs central server: peers

 IP address 1

 content
1 3
2) Alice queries for “Hey
2
Jude” 1

3) Alice requests file from
Bob
Alice


P2P: problems with centralized directory

Ì Single point of failure file transfer is
Ì Performance decentralized, but
bottleneck locating content is
Ì Copyright highly centralized
infringement


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


Gnutella: protocol
File transfer:
Ì Query message
HTTP
sent over existing TCP
connections
Query
Ì peers forward
QueryHit
Query message
y Qu
Ì QueryHit er ery
Qu Hi
t
sent over er
y
Qu
reverse
Query
path
QueryHit

Scalability: Qu
er
y
limited scope
flooding

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!


Exploiting heterogeneity: KaZaA

Ì 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.
Ì Group leader tracks the
content in all its o r d in a r y p e e r

children. 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


KaZaA: Querying
Ì Each file has a hash and a descriptor
Ì Client sends keyword query to its group
leader
Ì Group leader responds with matches:
 For each match: metadata, hash, IP address
Ì If group leader forwards query to other
group leaders, they respond with matches
Ì Client then selects files for downloading
 HTTP requests using hash as identifier sent to
peers holding desired file


KaZaA tricks
Ì Limitations on simultaneous uploads
Ì Request queuing
Ì Incentive priorities
Ì Parallel downloading


Ì 2.5 DNS server


Socket programming
Goal: learn how to build client/server application that
communicate using sockets

Socket API socket
Ì introduced in BSD4.1 UNIX,
a host-local,
1981
application-created,
Ì explicitly created, used, OS-controlled interface
released by apps (a “door”) into which
Ì client/server paradigm application process can
Ì two types of transport both send and
service via socket API: receive messages to/from
 unreliable datagram another application
process
 reliable, byte stream-
oriented


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


Socket programming with TCP
Client must contact server Ì When contacted by client,
Ì server process must first server TCP creates new
be running socket for server process to
Ì 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
Ì creating client-local TCP
clients (more in Chap 3)
socket
Ì specifying IP address, port application viewpoint
number of server process
TCP provides reliable, in-order
Ì When client creates
transfer of bytes (“pipe”)
socket: client TCP between client and server
establishes connection to
server TCP

Stream jargon
Ì A stream is a sequence of
characters that flow into
or out of a process.
Ì An input stream is
attached to some input
source for the process,
e.g., keyboard or socket.
Ì An output stream is
attached to an output
source, e.g., monitor or
socket.


Socket programming with TCP
keyboard monitor

Example client-server app:
1) client reads line from
standard input (inFromUser

inFromUser
input
stream
stream) , sends to server via Client
socket (outToServer Process
process
stream)
2) server reads line from socket
3) server converts line to
uppercase, sends back to

inFromServer
outToServer
output input
client stream stream

4) client reads, prints modified
line from socket client TCP
clientSocket
socket
(inFromServer stream)
TCP
socket

to network from network


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

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

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();

}
}

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


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


Ì 2.5 DNS server


Socket programming with UDP

UDP: no “connection” between
client and server
Ì no handshaking
Ì sender explicitly attaches application viewpoint
IP address and port of
destination to each packet UDP provides unreliable transfer
of groups of bytes (“datagrams”)
Ì 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


Client/server socket interaction: UDP
Server (running on hostid) Client

create socket,
port=x, for create socket,
clientSocket =
incoming request: DatagramSocket()
serverSocket =
DatagramSocket()
Create, address (hostid, port=x,
send datagram request
using clientSocket
read request from
serverSocket

write reply to
serverSocket
specifying client read reply from
host address, clientSocket
port number close
clientSocket


Example: Java client (UDP)
keyboard monitor

inFromUser
input
stream

Client
Process
Input: receives
process
packet (TCP
Output: sends received “byte
packet (TCP sent stream”)

receivePacket
sendPacket
“byte stream”) UDP UDP
packet packet

client UDP
clientSocket
socket UDP
socket

to network from network


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();

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();
}
}


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

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

server
Ì 2.5 DNS


Building a simple Web server
Ì handles one HTTP Ì after creating server,
request you can request file
Ì accepts the request using a browser (e.g.,
IE explorer)
Ì parses header
Ì see text for details
Ì obtains requested file
from server’s file
system
Ì creates HTTP response
message:
 header lines + file
Ì sends response to client


Chapter 2: Summary
Our study of network apps now complete!
Ì Application architectures Ì specific protocols:
 client-server  HTTP

 P2P  FTP

 hybrid  SMTP, POP, IMAP
 DNS
Ì application service
requirements: Ì socket programming
 reliability, bandwidth,
delay
Ì Internet transport
service model
 connection-oriented,
reliable: TCP
 unreliable, datagrams: UDP

Chapter 2: Summary
Most importantly: learned about protocols
Ì typical request/reply
Ì control vs. data msgs
message exchange:  in-band, out-of-band
 client requests info or
service
Ì centralized vs. decentralized
 server responds with
Ì stateless vs. stateful
data, status code Ì reliable vs. unreliable msg
transfer
Ì message formats:
Ì “complexity at network
 headers: fields giving
edge”
info about data
 data: info being
communicated


3rd edition chapter2

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