程序代写代做代考 Java python FTP database dns cache file system Chapter 2 Application Layer

Chapter 2 Application Layer
Computer
Networking: A Top
Down Approach
6th edition
Jim Kurose, Keith Ross Addison- Wesley March 2012
Application Layer 2-1

Chapter 2: outline
2.1 principles of network applications
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
 SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-2

Chapter 2: application layer
our goals:
 conceptual, implementation aspects of network application protocols
 transport-layer service models
 client-server paradigm
 peer-to-peer paradigm

learn about protocols by examining popular application-level protocols
 HTTP
 FTP
 SMTP / POP3 / IMAP  DNS
creating network applications
 socket API

Application Layer 2-3

Some network apps
 e-mail
 web
 text messaging
 remote login
 P2P file sharing
 multi-user network games
 streaming stored video (YouTube, Hulu, Netflix)
 voice over IP (e.g., Skype)  real-time video
conferencing
 social networking  search
…
…
Application Layer 2-4

Creating a network app
write programs that:
 run on (different) end systems
 communicate over network
 e.g., web server software communicates with browser software
no need to write software for network-core devices
 network-core devices do not run user applications
 applications on end systems allows for rapid app development, propagation
application
transport
network
data link
physical
application
transport
network
application
transport
network
data link physical
data link
physical
Application Layer 2-5

Application architectures
possible structure of applications:
 client-server
 peer-to-peer (P2P)
Application Layer 2-6

Client-server architecture
client/server
server:
 always-on host
 permanent IP address  data centers for scaling
clients:
 communicate with server  may be intermittently
connected
 may have dynamic IP addresses
 do not communicate directly with each other
Application Layer 2-7

P2P architecture
 no always-on server  arbitrary end systems
directly communicate
 peers request service from other peers, provide service in return to other peers
 self scalability – new peers bring new service capacity, as well as new service demands
 peers are intermittently connected and change IP addresses
 complex management
peer-peer
Application Layer 2-8

Processes communicating
process: program running within a host
 within same host, two processes communicate using inter-process communication (defined by OS)
clients, servers
client process: process that initiates communication
server process: process that waits to be contacted
 processes in different hosts
communicate by exchanging  aside: applications with P2P
messages
architectures have client processes & server processes
Application Layer 2-9

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 to deliver message to socket at receiving process
application
process
transport
network
link
physical
application
process
transport
network
link
physical
socket
Internet
controlled by app developer
controlled by OS
Application Layer 2-10

Addressing processes
 to receive messages, process must have identifier
 host device has unique 32- bit IP address
 Q: does IP address of host on which process runs suffice for identifying the process?
 A: 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
 to send HTTP message to gaia.cs.umass.edu web server:
 IP address: 128.119.245.12  port number: 80
 more shortly…
Application Layer 2-11

App-layer protocol defines
 types of messages exchanged,
 e.g., request, response
 message syntax:
 what fields in messages & how fields are delineated
 message semantics
 meaning of information
in fields
 rules for when and how processes send & respond to messages
open protocols:
 defined in RFCs
 allows for interoperability  e.g., HTTP, SMTP proprietary protocols:
 e.g., Skype
Application Layer 2-12

What transport service does an app need?
data integrity
 some apps (e.g., file transfer, web transactions) require
100% reliable data transfer
 other apps (e.g., audio) can tolerate some loss
timing
 some apps (e.g., Internet telephony, interactive games) require low delay to be “effective”
throughput
 some apps (e.g., multimedia) require minimum amount of throughput to be “effective”
 other apps (“elastic apps”) make use of whatever throughput they get
security
 encryption, data integrity, …
Application Layer 2-13

Transport service requirements: common apps
application
file transfer e-mail Web documents real-time audio/video
stored audio/video interactive games text messaging
data loss
no loss
no loss
no loss loss-tolerant
loss-tolerant loss-tolerant no loss
throughput
elastic
elastic
elastic
audio: 5kbps-1Mbps video:10kbps-5Mbps same as above
few kbps up elastic
time sensitive
no
no
no
yes, 100’s msec
yes, few secs yes, 100’s msec
yes and no
Application Layer 2-14

Internet transport protocols services
TCP service:
 reliable transport between sending and receiving process
 flow control: sender won’t overwhelm receiver
 congestion control: throttle sender when network overloaded
 does not provide: timing, minimum throughput guarantee, security
 connection-oriented: setup required between client and server processes
UDP service:
 unreliable data transfer between sending and receiving process
 does not provide: reliability, flow control, congestion control, timing, throughput guarantee, security, orconnection setup,
Q: why bother? Why is there a UDP?
Application Layer 2-15

Internet apps: application, transport protocols
application
e-mail remote terminal access
Web file transfer streaming multimedia
Internet telephony
application layer protocol
SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959]
HTTP (e.g., YouTube), RTP [RFC 1889]
SIP, RTP, proprietary (e.g., Skype)
underlying transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
TCP or UDP
Application Layer 2-16

Securing TCP
TCP & UDP
 no encryption
 cleartext passwds sent into socket traverse Internet in cleartext
SSL/TLS
 provides encrypted TCP connection
 data integrity
 end-point authentication
SSL/TLS is at app layer
 Apps use SSL libraries, which “talk” to TCP
SSL socket API
 cleartext passwds sent into socket traverse Internet encrypted
 TLS is a newer version of SSL, it still operates in the application layer
Application Layer 2-17

Regular Web Connection
Application Layer 2-18

SSL/TLS Web Connection
Application Layer 2-19

Chapter 2: outline
2.1 principles of network applications
 app architectures  app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
 SMTP, POP3, IMAP 2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-20

Web and HTTP
First, a review…
 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, e.g.,
www.someschool.edu/someDept/pic.gif
host name
path name
Application Layer 2-21

HTTP overview
HTTP: hypertext transfer protocol
 Web’s application layer protocol
 client/server model
 client: browser that requests, receives, (using HTTP protocol) and “displays” Web objects
 server: Web server sends (using HTTP protocol) objects in response to requests
PC running Firefox browser
server running
Apache Web server
iphone running Safari browser
Application Layer 2-22

HTTP overview (continued)
uses TCP:
 client initiates TCP connection (creates socket) to server, port 80
 server accepts TCP connection from client
 HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)
 TCP connection closed
HTTP is “stateless”
 server maintains no information about past client requests
aside
 past history (state) must be maintained
 if server/client crashes, their views of “state” may be inconsistent, must be reconciled
protocols that maintain “state” are complex!
Application Layer 2-23

HTTP connections
non-persistent HTTP
 at most one object sent over TCP connection
 connection then closed
 downloading multiple objects required multiple connections
persistent HTTP
 multiple objects can be sent over single TCP connection between client, server
Application Layer 2-24

Non-persistent HTTP
suppose user enters URL:
www.someSchool.edu/someDepartment/home.index
1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80
2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index
(contains text, references to 10
jpeg images)
1b. HTTP server at host www.someSchool.edu waiting for TCP connection at port 80. “accepts” connection, notifying client
3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket
time
Application Layer 2-25

Non-persistent HTTP (cont.)
5. HTTP client receives response message containing html file, displayshtml. Parsinghtmlfile, finds 10 referenced jpeg objects
time
4. HTTP server closes TCP connection.
6. Steps 1-5 repeated for each of 10 jpeg objects
Application Layer 2-26

Non-persistent HTTP: response time
RTT (definition): time for a small packet to travel from client to server and back
HTTP response time:
 one RTT to initiate TCP connection
 one RTT for HTTP request and first few bytes of HTTP response to return
initiate TCP connection
RTT
request file
RTT
time to transmit file
time
 file transmission time file
 non-persistent HTTP response time =
2RTT+ file transmission time
received
time
Application Layer 2-27

Persistent HTTP
non-persistent HTTP issues:
 requires 2 RTTs per object
 OS overhead for each TCP
connection
 browsers often open parallel TCP connections to fetch referenced objects
persistent HTTP:
 server leaves connection open after sending response
 subsequent HTTP messages between same 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
Application Layer 2-28

HTTP request message
 two types of HTTP messages: request, response  HTTP request message:
 ASCII (human-readable format)
request line
(GET, POST, HEAD commands)
header lines
carriage return, line feed at start
of line indicates end of header lines
carriage return character line-feed character
GET /index.html HTTP/1.1\r\n
Host: www-net.cs.umass.edu\r\n
User-Agent: Firefox/3.6.10\r\n
Accept: text/html,application/xhtml+xml\r\n Accept-Language: en-us,en;q=0.5\r\n Accept-Encoding: gzip,deflate\r\n Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n Keep-Alive: 115\r\n
Connection: keep-alive\r\n \r\n
Application Layer 2-29

HTTP request message: general format
method
sp
URL
sp
version
cr
lf
header field name
value
cr
lf
~~ ~~
request line
header lines
header field name
value
cr
lf
cr
lf
~ entity body ~~
body
~
Application Layer 2-30

Uploading form input
POST method:
 web page often includes form input
 input is uploaded to server in entity body
URL method:
 uses GET method
 input is uploaded in URL
field of request line:
www.somesite.com/animalsearch?monkeys&banana
Application Layer 2-31

Method types
HTTP/1.0:
 GET  POST  HEAD
 asks server to leave requested object out of response
HTTP/1.1:
 GET, POST, HEAD  PUT
 uploads file in entity body to path specified in URL field
 DELETE
 deletes file specified in the URL field
Application Layer 2-32

HTTP response message
status line (protocol status code status phrase)
header lines
HTTP/1.1 200 OK\r\n
Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n Server: Apache/2.0.52 (CentOS)\r\n Last-Modified: Tue, 30 Oct 2007 17:00:02
GMT\r\n
ETag: “17dc6-a5c-bf716880″\r\n Accept-Ranges: bytes\r\n
Content-Length: 2652\r\n
Keep-Alive: timeout=10, max=100\r\n Connection: Keep-Alive\r\n
Content-Type: text/html; charset=ISO-8859-
1\r\n \r\n
data data data data data …
data, e.g., requested HTML file
Application Layer 2-33

HTTP response status codes
 status code appears in 1st line in server-to- client response message.
 some sample codes:
200 OK
 request succeeded, requested object later in this msg 301 Moved Permanently
 requested object moved, new location specified later in this msg (Location:)
400 Bad Request
 request msg not understood by server 404 Not Found
 requested document not found on this server 505 HTTP Version Not Supported
Application Layer 2-34

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
Host: cis.poly.edu
by typing this in (hit carriage return twice), you send
this minimal (but complete) GET request to HTTP server
3. look at response message sent by HTTP server!
(or use Wireshark to look at captured HTTP request/response)
Application Layer 2-35

User-server state: cookies
example:
 Susan always access Internet from PC
many Web sites use cookies
four components:
1) cookie header line of HTTP response message
2) cookie header line in next HTTP request message
3) cookie file kept on user’s host, managed by user’s browser
4) back-end database at Web site
 
visits specific e-commerce site for first time
when initial HTTP requests arrives at site, site creates:
 unique ID
 entry in backend database for ID
Application Layer 2-36

Cookies: keeping “state” (cont.)
client
ebay 8734
cookie file
ebay 8734 amazon 1678
server
Amazon server creates ID 1678 for user
cookie- specific action
access
cookie- specific action
usual http request msg
usual http response
set-cookie: 1678
backend entry database
access
create
usual http request msg
cookie: 1678
usual http response msg
one week later:
ebay 8734 amazon 1678
usual http request msg
cookie: 1678
usual http response msg
Application Layer 2-37

Cookies (continued)
what cookies can be used for:
 authorization
 shopping carts
 recommendations
 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
cookies and privacy: aside  cookies permit sites to
learn a lot about you
 you may supply name and e-mail to sites
Application Layer 2-38

Web caches (proxy server)
goal: satisfy client request without involving origin server  user sets browser: Web
accesses via cache
 browser sends all HTTP requests to cache
 object in cache: cache returns object
 else cache requests object from origin server, then returns object to client
proxy server
client
origin server
client
origin server
Application Layer 2-39

More about Web caching
 cache acts as both client and server
 server for original requesting client
 client to origin server
 typically cache is installed by ISP (university, company, residential ISP)
why Web caching?
 reduce response time for client request
 reduce traffic on an institution’s access link
 Internet dense with caches: enables “poor” content providers to effectively deliver content (so too does P2P file sharing)
Application Layer 2-40

Caching example:
assumptions:
 avg object size: 100K bits
 avg request rate from browsers to
origin servers:15/sec
 avg data rate to browsers: 1.50 Mbps
 RTT from institutional router to any origin server: 2 sec
 access link rate: 1.54 Mbps
consequences:
 LAN utilization: 15% problem!
 access link utilization = 99%
 total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + usecs
origin servers
institutional network
1 Gbps LAN
public Internet
1.54 Mbps access link
Application Layer 2-41

Caching example: fatter access link
assumptions:
 avg object size: 100K bits
 avg request rate from browsers to
origin servers:15/sec
 avg data rate to browsers: 1.50 Mbps
 RTT from institutional router to any origin server: 2 sec
 access link rate: 1.54 Mbps 154 Mbps
origin servers
public Internet
consequences:
 L AN utilization: 15%
 access link utilization = 99%
1.54 Mbps access link
154 Mbps
 total delay = Internet delay + access
delay + LAN delay
= 2 sec + minutes + usecs
msecs
9.9%
institutional network
1 Gbps LAN
Cost: increased access link speed (not cheap!)
Application Layer 2-42

Caching example: install local cache
assumptions:
 avg object size: 100K bits
 avg request rate from browsers to
origin servers:15/sec
 avg data rate to browsers: 1.50 Mbps
 RTT from institutional router to any origin server: 2 sec
 access link rate: 1.54 Mbps
consequences:
origin servers
?
 total delay = In?ternet delay + access
public Internet
 L AN utilization: 15%
 access link utilization = 100%
institutional network
1.54 Mbps access link
1 Gbps LAN
local web cache
delay + LAN delay
How to compute link
= 2 sec + minutes + usecs
utilization, delay?
Cost: web cache (cheap!)
Application Layer 2-43

Caching example: install local cache
Calculating access link utilization, delay with cache:
suppose cache hit rate is 0.4  40% requests satisfied at cache,
60% requests satisfied at origin
access link utilization:
 60% of requests use access link
 data rate to browsers over access link = 0.6*1.50 Mbps = .9 Mbps
 utilization = 0.9/1.54 = .58 total delay
 = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache)
 = 0.6 (2.01) + 0.4 (~msecs)
 = ~ 1.2 secs
 less than with 154 Mbps link (and cheaper too!)
origin servers
institutional network
1 Gbps LAN
local web cache
public Internet
1.54 Mbps access link
Application Layer 2-44

Conditional GET
 Goal: don’t send object if cache has up-to-date cached version
 no object transmission delay
 lower link utilization
 cache: specify date of cached copy in HTTP request
If-modified-since:

 server: response contains no object if cached copy is up-to-date:
HTTP/1.0 304 Not Modified
client
server
object not
modified before
HTTP request msg
If-modified-since:
HTTP response
HTTP/1.0 304 Not Modified
HTTP request msg
If-modified-since:
object modified
after
HTTP response
HTTP/1.0 200 OK
Application Layer 2-45

Chapter 2: outline
2.1 principles of network applications
 app architectures  app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
 SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-46

FTP: the file transfer protocol
FTP
user interface
FTP client
FTP server
user at host
file transfer
local file system
remote file 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
Application Layer 2-47

FTP: separate control, data connections
 FTP client contacts FTP server at port 21, using TCP
 client authorized over control connection
 client browses remote directory, sends commands over control connection
 when server receives file transfer command, server opens 2nd TCP data connection (for file) to client
 after transferring one file, server closes data connection
FTP client
TCP control connection, server port 21
TCP data connection, FTP
server port 20
server
 server opens another TCP data connection to transfer another file
 control connection: “out of band”
 FTP server maintains “state”: current directory, earlier authentication
Application Layer 2-48

FTP commands, responses
sample commands:
 sent as ASCII text over control channel
 USER username
 PASS password
 LIST return list of file in current directory
 RETR filename retrieves (gets) file
 STOR filename stores (puts) file onto remote host
sample return codes
 status code and phrase (as in HTTP)
 331 Username OK, password required
 125 data connection already open; transfer starting
 425 Can’t open data connection
 452 Error writing file
Application Layer 2-49

Chapter 2: outline
2.1 principles of network applications
 app architectures  app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
 SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-50

Electronic mail
Three major components:
 user agents
 mail servers
 simple mail transfer protocol: SMTP
User Agent
 a.k.a. “mail reader”
 composing, editing, reading
mail messages
 e.g., Outlook, Thunderbird, iPhone mail client
 outgoing, incoming messages stored on server
SMTP
SMTP
SMTP
outgoing message queue
user mailbox
user agent
mail server
user agent
mail server
user agent
user agent
mail server
user agent
user agent
Application Layer 2-51

Electronic mail: mail servers
mail servers:
 mailbox contains incoming messages for user
 message queue of outgoing (to be sent) mail messages
 SMTP protocol between mail servers to send email messages
 client: sending mail server
 “server”: receiving mail server
SMTP
SMTP
user agent
mail server
user agent
mail server
user agent
SMTP
user agent
mail server
user agent
user agent
Application Layer 2-52

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 (like HTTP, FTP)  commands: ASCII text
 response: status code and phrase
 messages must be in 7-bit ASCI
Application Layer 2-53

Scenario: Alice sends message to Bob
1) Alice uses UA to compose message “to” bob@someschool.edu
2) Alice’s UA sends message to her mail server; message placed in message queue
3) client side of SMTP opens TCP connection with Bob’s mail server
4) SMTP client sends Alice’s message over the TCP connection
5) Bob’s mail server places the message in Bob’s mailbox
6) Bob invokes his user agent to read message
user agent
1 user agent
2
4
6
mail server
mail server
3
Alice’s mail server
5
Bob’s mail server
Application Layer 2-54

Sample SMTP interaction
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM:
S: 250 alice@crepes.fr… Sender ok
C: RCPT TO:
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
Application Layer 2-55

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)
Application Layer 2-56

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
comparison with HTTP:
 HTTP: pull
 SMTP: push
 both have ASCII command/response interaction, status codes
 HTTP: each object encapsulated in its own response msg
 SMTP: multiple objects sent in multipart msg
Application Layer 2-57

Mail message format
SMTP: protocol for exchanging email msgs
RFC 822: standard for text message format:
 header lines, e.g.,  To:
 From:
 Subject:
different from SMTP MAIL FROM, RCPT TO: commands!
 Body: the “message”  ASCII characters only
blank line
header
body
Application Layer 2-58

Mail access protocols
SMTP SMTP
sender’s mail server
mail access protocol
(e.g., POP, IMAP)
user agent
user agent
 SMTP: delivery/storage to receiver’s server  mail access protocol: retrieval from server
 POP: Post Office Protocol [RFC 1939]: authorization, download
 IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored msgs on server
 HTTP: gmail, Hotmail, Yahoo! Mail, etc.
receiver’s mail server
Application Layer 2-59

POP3 protocol
authorization phase
 client commands:
 user: declare username  pass: password
 server responses  +OK
 -ERR
transaction phase, client:
 list: list message numbers
 retr: retrieve message by number
 dele:delete
 quit
S: +OK POP3 server ready C: user bob
S: +OK
C: pass hungry
S: +OK user successfully logged on
C: list
S: 1 498
S: 2 912
S: .
C: retr 1
S: S: .
C: dele 1
C: retr 2
S: S: .
C: dele 2
C: quit
S: +OK POP3 server signing off
Application Layer 2-60

POP3 (more) and IMAP
more about POP3
IMAP
 keeps all messages in one place: at server
 allows user to organize messages in folders
 keeps user state across sessions:
 names of folders and mappings between message IDs and folder name
 previous example uses POP3 “download and delete” mode
 Bob cannot re-read e- mail if he changes client
 POP3 “download-and- keep”: copies of messages on different clients
 POP3 is stateless across sessions
Application Layer 2-61

Chapter 2: outline
2.1 principles of network applications
 app architectures  app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
 SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-62

DNS: domain name system
people: many identifiers:
 SSN, name, passport #
Internet hosts, routers:
 IP address (32 bit) – used for addressing datagrams
 “name”, e.g., www.yahoo.com – used by humans
Q: how to map between IP address and name, and vice versa ?
Domain Name System:
 distributed database implemented in hierarchy of many name servers
 application-layer protocol: hosts, name servers communicate to resolve names (address/name translation)
 note: core Internet function, implemented as application- layer protocol
 complexity at network’s “edge”
Application Layer 2-63

DNS: services, structure
DNS services
 hostname to IP address translation
 host aliasing
 canonical, alias names
 mail server aliasing  load distribution
 replicated Web servers: many IP addresses correspond to one name
why not centralize DNS?
 single point of failure
 traffic volume
 distant centralized database  maintenance
A: doesn’t scale!
Application Layer 2-64

DNS: a distributed, hierarchical database
com DNS servers
yahoo.com DNS servers
Root DNS Servers
……
org DNS servers
pbs.org DNS servers
edu DNS servers
poly.edu umass.edu DNS serversDNS servers
amazon.com DNS servers
client wants IP for www.amazon.com; 1st approx:
 client queries 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
Application Layer 2-65

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
c. Cogent, Herndon, VA (5 other sites) d. U Maryland College Park, MD
h. ARL Aberdeen, MD
j. Verisign, Dulles VA (69 other sites )
e. NASA Mt View, CA
f. Internet Software C.
Palo Alto, CA (and 48 other sites)
a. Verisign, Los Angeles CA (5 other sites)
b. USC-ISI Marina del Rey, CA l. ICANN Los Angeles, CA
k. RIPE London (17 other sites)
i. Netnod, Stockholm (37 other sites)
m. WIDE Tokyo (5 other sites)
13 root name “servers” worldwide
(41 other sites)
g. US DoD Columbus, OH (5 other sites)
Application Layer 2-66

TLD, authoritative servers
top-level domain (TLD) servers:
 responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp
 Network Solutions maintains servers for .com TLD
 Educause for .edu TLD
authoritative DNS servers:
 organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts
 can be maintained by organization or service provider Application Layer 2-67

Local DNS 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
 has local cache of recent name-to-address translation pairs (but may be out of date!)
 acts as proxy, forwards query into hierarchy
Application Layer 2-68

DNS name resolution example
 host at cis.poly.edu wants IP address for gaia.cs.umass.edu
iterated query:
 contacted server replies with name of server to contact
 “I don’t know this name, but ask this server”
root DNS server
2
3
4 5
TLD DNS server
local DNS server
dns.poly.edu
1 8
requesting host
cis.poly.edu
7 6
authoritative DNS server
dns.cs.umass.edu
gaia.cs.umass.edu
Application Layer 2-69

DNS name resolution example
recursive query:
 puts burden of name resolution on contacted name server
 heavy load at upper levels of hierarchy?
root DNS server
3 7
2
6
local DNS server
dns.poly.edu
18
requesting host
cis.poly.edu
TLD DNS server
4
authoritative DNS server
dns.cs.umass.edu
gaia.cs.umass.edu
5
Application Layer 2-70

DNS: caching, updating records
 once (any) name server learns mapping, it caches mapping
 cache entries timeout (disappear) after some time (TTL)  TLD servers typically cached in local name servers
• thus root name servers not often visited
 cached entries may be out-of-date (best effort name-to-address translation!)
 if name host changes IP address, may not be known Internet-wide until all TTLs expire
 update/notify mechanisms proposed IETF standard  RFC 2136
Application Layer 2-71

DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)
type=A
 name is hostname
 value is IP address
type=NS
 name is domain (e.g., foo.com)
 value is hostname of authoritative name server for this domain
type=CNAME
 name is alias name for some “canonical” (the real) name
 www.ibm.com is really servereast.backup2.ibm.com
 value is canonical name type=MX
 value is name of mailserver associated with name
Application Layer 2-72

DNS protocol, messages
 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 bytes
2 bytes
identification
# questions
# authority RRs
questions (variable # of questions)
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
flags
# answer RRs
# additional RRs
Application Layer 2-73

DNS protocol, messages
2 bytes
2 bytes
identification
# questions
# authority RRs
questions (variable # of questions)
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
flags
# answer RRs
# additional RRs
name, type fields for a query
RRs in response to query
records for authoritative servers
additional “helpful” info that may be used
Application Layer 2-74

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
Application Layer 2-75

Attacking DNS
DDoS attacks
 Bombard root servers with traffic
 Not successful to date
 Traffic Filtering
 Local DNS servers cache IPs of TLD servers, allowing root server bypass
 Bombard TLD servers
 Potentially more dangerous
Redirect attacks
 Man-in-middle
 Intercept queries
 DNS poisoning
 Send bogus relies to DNS server, which caches
Exploit DNS for DDoS
 Send queries with spoofed source address: target IP
 Requires amplification Application Layer 2-76

Chapter 2: outline
2.1 principles of network applications
 app architectures  app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
 SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-77

Pure P2P architecture
 no always-on server  arbitrary end systems
directly communicate
 peers are intermittently connected and change IP addresses
examples:
 file distribution (BitTorrent)
 Streaming (KanKan)  VoIP (Skype)
Application Layer 2-78

File distribution: client-server vs P2P
Question: how much time to distribute file (size F) from one server to N peers?
 peer upload/download capacity is limited resource us: server upload
file, size F
capacity
us
u d
1 1 u2 d2
network (with abundant bandwidth)
di: peer i download capacity
di ui
ui: peer i upload capacity
server
uN dN
Application Layer 2-79

File distribution time: client-server
 server transmission: must sequentially send (upload) N F file copies:
 time to send one copy: F/us
 time to send N copies: NF/us
 client: each client must download file copy
 dmin = min client download rate  min client download time: F/dmin
us
network
di
ui
time to distribute F to N clients using
client-server approach
Dc-s > max{NF/us,,F/dmin}
increases linearly in N
Application Layer 2-80

File distribution time: P2P
 server transmission: must
upload at least one copy F
 time to send one copy: F/us
 client: each client must download file copy
us
di network ui
 min client download time: F/dmin
 clients: as aggregate must download NF bits
 max upload rate (limting max download rate) is us + Sui
time to distribute F to N clients using
P2P approach
DP2P > max{F/us,,F/dmin,,NF/(us + Sui)}
increases linearly in N …
… but so does this, as each peer brings service capacity
Application Layer 2-81

Client-server vs. P2P: example
client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
3.5 3 2.5 2 1.5 1 0.5 0
P2P Client-Server
0 5 10 15 20 25 30 35 N
Application Layer 2-82
Minimum Distribution Time

P2P file distribution: BitTorrent
 file divided into 256Kb chunks
 peers in torrent send/receive file chunks
tracker: tracks peers participating in torrent
Alice arrives …
… obtains list
of peers from tracker
… and begins exchanging
file chunks with peers in torrent
torrent: group of peers exchanging chunks of a file
Application Layer 2-83

P2P file distribution: BitTorrent
 peer joining torrent:
 has no chunks, but will accumulate them over time from other peers
 registers with tracker to get list of peers, connects to subset of peers (“neighbors”)
 while downloading, peer uploads chunks to other peers
 peer may change peers with whom it exchanges chunks
 churn: peers may come and go
 once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent
Application Layer 2-84

BitTorrent: requesting, sending file chunks
requesting chunks:
 at any given time, different peers have different subsets of file chunks
 periodically, Alice asks each peer for list of chunks that they have
 Alice requests missing chunks from peers, rarest first
sending chunks: tit-for-tat
 Alice sends chunks to those four peers currently sending her chunks at highest rate
 other peers are choked by Alice (do not receive chunks from her)
 re-evaluate top 4 every10 secs
 every 30 secs: randomly select another peer, starts sending chunks
 “optimistically unchoke” this peer  newly chosen peer may join top 4
Application Layer 2-85

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
higher upload rate: find better trading partners, get file faster !
Application Layer 2-86

Distributed Hash Table (DHT)
 Hash table
 DHT paradigm
 Circular DHT and overlay networks  Peer churn

Simple Database
Simple database with(key, value) pairs:
• key: human name; value: social security #
Key
Value
John Washington
132-54-3570
Diana Louise Jones
761-55-3791
Xiaoming Liu
385-41-0902
Rakesh Gopal
441-89-1956
Linda Cohen
217-66-5609
…….
………
Lisa Kobayashi
177-23-0199
• key: movie title; value: IP address

Hash Table
• More convenient to store and search on numerical representation of key
• key = hash(original key)
Original Key
Key
Value
John Washington
8962458
132-54-3570
Diana Louise Jones
7800356
761-55-3791
Xiaoming Liu
1567109
385-41-0902
Rakesh Gopal
2360012
441-89-1956
Linda Cohen
5430938
217-66-5609
…….
………
Lisa Kobayashi
9290124
177-23-0199

Distributed Hash Table (DHT)
 Distribute (key, value) pairs over millions of peers  pairs are evenly distributed over peers
 Any peer can query database with a key
 database returns value for the key
 To resolve query, small number of messages exchanged among peers
 Each peer only knows about a small number of other peers
 Robust to peers coming and going (churn)

Assign key-value pairs to peers
 rule: assign key-value pair to the peer that has the closest ID.
 convention: closest is the immediate successor of the key.
 e.g., ID space {0,1,2,3,…,63}
 suppose 8 peers: 1,12,13,25,32,40,48,60  If key = 51, then assigned to peer 60
 If key = 60, then assigned to peer 60
 If key = 61, then assigned to peer 1

•
Circular DHT
each peer only aware of immediate successor and predecessor.
1
12
13 25
60
48
40
32
“overlay network”

Resolving a query
1
What is the value associated with key 53
12
value
60
48
O(N) messages
on avgerage to resolve query, when there
are N peers
40
13
25 32
?

Circular DHT with shortcuts
1
value
What is the value for key 53
13 25
12
60
48
40
32
• each peer keeps track of IP addresses of predecessor, successor, short cuts.
• reduced from 6 to 3 messages.
• possible to design shortcuts with O(log N) neighbors, O(log N) messages in query

Peer churn
handling peer churn:
peers may come and go (churn) each peer knows address of its
two successors
each peer periodically pings its
1
3
15
12
10
8
4 two successors to check aliveness
5
example: peer 5 abruptly leaves
if immediate successor leaves, choose next successor as new immediate successor

Peer churn
handling peer churn:
peers may come and go (churn) each peer knows address of its
two successors
each peer periodically pings its
15
12
10
1
3
8
example: peer 5 abruptly leaves
4 two successors to check aliveness
if immediate successor leaves, choose next successor as new immediate successor
peer 4 detects peer 5’s departure; makes 8 its immediate successor
 4 asks 8 who its immediate successor is; makes 8’s immediate successor its second successor.

Chapter 2: outline
2.1 principles of network applications
 app architectures  app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
 SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-97

Socket programming
goal: learn how to build client/server applications that communicate using sockets
socket: door between application process and end- end-transport protocol
application
process
transport
network
link
physical
application
process
transport
network
link
physical
socket
Internet
controlled by app developer
controlled by OS
Application Layer 2-98

Socket programming
Two socket types for two transport services:
 UDP: unreliable datagram
 TCP: reliable, byte stream-oriented
Application Example:
1. Client reads a line of characters (data) from its keyboard and sends the data to the server.
2. The server receives the data and converts characters to uppercase.
3. The server sends the modified data to the client.
4. The client receives the modified data and displays the line on its screen.
Application Layer 2-99

Socket programming with UDP
UDP: no “connection” between client & server
 no handshaking before sending data
 sender explicitly attaches IP destination address and port # to each packet
 rcvr extracts sender IP address and port# from received packet
UDP: transmitted data may be lost or received out-of-order
Application viewpoint:
UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server
Application Layer 2-100

Client/server socket interaction: UDP
server (running on serverIP) create socket, port= x:
serverSocket = socket(AF_INET,SOCK_DGRAM)
read datagram from
serverSocket
write reply to
serverSocket
specifying client address, port number
client
create socket:
clientSocket = socket(AF_INET,SOCK_DGRAM)
Create datagram with server IP and port=x; send datagram via clientSocket
read datagram from
clientSocket
close
clientSocket
Application 2-101

Example app: UDP client
include Python’s socket library
create UDP socket for server
get user keyboard input
Attach server name, port to message; send into socket
read reply characters from socket into string
print out received string and close socket
Python UDPClient
from socket import *
serverName = ‘hostname’
serverPort = 12000
clientSocket = socket(socket.AF_INET,
socket.SOCK_DGRAM) message = raw_input(’Input lowercase sentence:’)
clientSocket.sendto(message,(serverName, serverPort)) modifiedMessage, serverAddress =
clientSocket.recvfrom(2048) print modifiedMessage
clientSocket.close()
Application Layer 2-102

Example app: UDP server
create UDP socket
bind socket to local port number 12000
loop forever
Read from UDP socket into message, getting client’s address (client IP and port)
send upper case string back to this client
Python UDPServer
from socket import *
serverPort = 12000
serverSocket = socket(AF_INET, SOCK_DGRAM) serverSocket.bind((”, serverPort))
print “The server is ready to receive”
while 1:
message, clientAddress = serverSocket.recvfrom(2048) modifiedMessage = message.upper() serverSocket.sendto(modifiedMessage, clientAddress)
Application Layer 2-103

Socket programming with TCP
client must contact server
 server process must first be running
 server must have created socket (door) that welcomes client’s contact
client contacts server by:
 Creating TCP socket, specifying IP address, port number of server process
 when client creates socket: client TCP establishes connection to server TCP
 when contacted by client, server TCP creates new socket for server process to communicate with that particular client
 allows server to talk with multiple clients
 source port numbers used to distinguish clients (more in Chap 3)
application viewpoint:
TCP provides reliable, in-order byte-stream transfer (“pipe”) between client and server
Application Layer 2-104

Client/server socket interaction: TCP
server (running on hostid)
create socket,
port=x, for incoming
request:
serverSocket = socket()
wait for incoming connection request connectionSocket = serverSocket.accept()
read request from
connectionSocket write reply to
connectionSocket
close
connectionSocket
client
TCP connection setup
create socket,
connect to hostid, port=x clientSocket = socket()
send request using
clientSocket
read reply from
clientSocket
close
clientSocket
Application Layer 2-105

Example app: TCP client
create TCP socket for server, remote port 12000
Python TCPClient
from socket import *
serverName = ’servername’
serverPort = 12000
clientSocket = socket(AF_INET, SOCK_STREAM) clientSocket.connect((serverName,serverPort)) sentence = raw_input(‘Input lowercase sentence:’) clientSocket.send(sentence)
modifiedSentence = clientSocket.recv(1024)
print ‘From Server:’, modifiedSentence clientSocket.close()
No need to attach server name, port
Application Layer 2-106

Example app: TCP server
create TCP welcoming socket
server begins listening for incoming TCP requests
loop forever
server waits on accept()
for incoming requests, new socket created on return
read bytes from socket (but not address as in UDP)
close connection to this client (but not welcoming socket)
Python TCPServer
from socket import *
serverPort = 12000
serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((‘’,serverPort)) serverSocket.listen(1)
print ‘The server is ready to receive’
while 1:
connectionSocket, addr = serverSocket.accept()
sentence = connectionSocket.recv(1024) capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence) connectionSocket.close()
Application Layer 2-107

Chapter 2: summary
our study of network apps now complete!
 application architectures  client-server
 P2P
 application service requirements:
 reliability, bandwidth, delay
 Internet transport service
model
 connection-oriented, reliable: TCP
 unreliable, datagrams: UDP
 specific protocols:  HTTP
 FTP
 SMTP, POP, IMAP
 DNS
 P2P: BitTorrent, DHT
 socket programming: TCP, UDP sockets
Application Layer 2-108

Chapter 2: summary
most importantly: learned about protocols!
 typical request/reply message exchange:
 client requests info or service
 server responds with data, status code
 message formats:
 headers: fields giving
info about data
 data: info being communicated
important themes:
 control vs. data msgs
 in-band, out-of-band
 centralized vs. decentralized
 stateless vs. stateful
 reliable vs. unreliable msg transfer
 “complexity at network edge”
Application Layer 2-109

Chapter 1 Additional Slides
Introduction 1-110

packet analyzer
packet capture (pcap)
application (www browser,
email client)
application OS
Transport (TCP/UDP)
Network (IP)
Link (Ethernet)
copy of all Ethernet frames sent/receive d
Physical