CS455 Introduction to Computer Networks

1
CS455 Introduction to Computer Networks
WSU Vancouver

• Dr. Wenzhan Song
• Assistant Professor, Computer Science

2
Introduction review

• Physical overview of Internet
• Physical architecture
• Network Edge - Internet access technologies
• Residential access dialup, ADSL, cable, WiMax
• Company access LAN, WLAN
• Network Core Switching technologies
• Circuit switching
• Packet switching
• Software overview of Internet
• Software architecture layering
• The OSI and TCP/IP Reference Models comparison
• Internet history
• Network standardization body

3
Course roadmap

• Introduction
• Application Layer WWW, FTP, email, DNS,
  multimedia
• Transport Layer reliable end-end data transfer
  principles, UDP, TCP
• Network Layer routing, congestion control, QoS
• Data Link Layer framing, error control, flow
  control
• Medium Access Control (MAC) Layer
  multiple-access, channel allocation
• Physical Layer wired, wireless, satellite
• Other Topics network security, social issues,
  hot topics, research directions

4
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
• P2P
• Multimedia systems

5
Application layer Roadmap

• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail
• SMTP, POP3, IMAP
• DNS
• P2P file sharing
• Multimedia network systems

6
Some network apps

• E-mail
• Web
• Instant messaging
• Remote login
• P2P file sharing
• Multi-user network games
• Streaming stored video clips

• Internet telephone
• Real-time video conference
• Massive parallel computing

7
Creating a network app

• Write programs that
• run on different end systems and
• communicate over a network.
• e.g., Web Web server software communicates with
  browser software
• No software written for devices in network core
• Network core devices do not function at app layer
• This design allows for rapid app development

8
Application architectures

• Client-server
• Peer-to-peer (P2P)
• Hybrid of client-server and P2P

9
Client-server archicture

• 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

10
Pure P2P architecture

• no always on server
• arbitrary end systems directly communicate
• peers are intermittently connected and change IP
  addresses
• example Gnutella
• Highly scalable
• But difficult to manage

11
Hybrid of client-server and P2P

• Napster
• File transfer is P2P
• File search is 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 is centralized
• User registers its IP address with central server
  when it comes online
• User contacts central server to find IP addresses
  of buddies

12
Processes communicating

• Client process process that initiates
  communication
• Server process process that waits to be
  contacted

• Process program running within a host.
• within same host, two processes communicate using
  inter-process communication (defined by OS).
• processes in different hosts communicate by
  exchanging messages

• Note applications with P2P architectures have
  client processes server processes

13
Sockets

• process sends/receives messages to/from its
  socket
• socket analogous to door
• sending process shoves message out door
• sending process relies on transport
  infrastructure on other side of door which brings
  message to socket at receiving process

controlled by app developer
Internet
controlled by OS

• API (1) choice of transport protocol (2)
  ability to fix a few parameters (socket
  programming)

14
Addressing processes

• For a process to receive messages, it must have
  an identifier
• A host has a unique 32-bit IP address
• Q does the IP address of the host on which the
  process runs suffice for identifying the process?
• Answer No, many processes can be running on same
  host

• Identifier includes both the IP address and port
  numbers associated with the process on the host.
• Example port numbers
• HTTP server 80
• Mail server 25
• More on this later

15
App-layer protocol defines

• Types of messages exchanged, e.g., request
  response messages
• Syntax of message types what fields in messages
  how fields are delineated
• Semantics of the fields, i.e., meaning of
  information in fields
• Rules for when and how processes send respond
  to messages

• Public-domain protocols
• defined in RFCs
• allows for interoperability
• e.g., HTTP, SMTP
• Proprietary protocols
• e.g., KaZaA

16
What transport service does an app need?

• Data loss
• some apps (e.g., audio) can tolerate some loss
• other apps (e.g., file transfer, telnet) require
  100 reliable data transfer

• Bandwidth
• some apps (e.g., multimedia) require minimum
  amount of bandwidth to be effective
• other apps (elastic apps) make use of whatever
  bandwidth they get

• Timing
• some apps (e.g., Internet telephony, interactive
  games) require low delay to be effective

17
Transport service requirements of common apps
Application file transfer e-mail Web
documents real-time audio/video stored
audio/video interactive games instant messaging
Time Sensitive no no no yes, 100s msec yes,
few secs yes, 100s msec yes and no
Bandwidth elastic elastic elastic audio
5kbps-1Mbps video10kbps-5Mbps same as above few
kbps up elastic
Data loss no loss no loss no loss loss-tolerant
loss-tolerant loss-tolerant no loss
18
Internet transport protocols services

• TCP service
• connection-oriented setup required between
  client and server processes
• reliable transport between sending and receiving
  process
• flow control sender wont overwhelm receiver
• congestion control throttle sender when network
  overloaded
• does not provide timing, minimum bandwidth
  guarantees

• UDP service
• unreliable data transfer between sending and
  receiving process
• does not provide connection setup, reliability,
  flow control, congestion control, timing, or
  bandwidth guarantee
• Q why bother? Why is there a UDP?

19
Internet apps application, transport protocols
Application layer protocol SMTP RFC
2821 Telnet RFC 854 HTTP RFC 2616 FTP RFC
959 proprietary (e.g. RealNetworks) proprietary (
e.g., Dialpad)
Underlying transport protocol TCP TCP TCP TCP TCP
or UDP typically UDP
Application e-mail remote terminal access Web
file transfer streaming multimedia Internet
telephony
20
Application layer Roadmap

• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail
• SMTP, POP3, IMAP
• DNS
• P2P file sharing
• Multimedia network systems

21
Architectural Overview of Web
22
Architectural Overview (2)

• The parts of the Web model.

23
The Server Side

• A multithreaded Web server with a front end and
  processing modules.

24
The Server Side (2)

• A server farm.

25
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

26
Simple HTML file

• lthtmlgt
• ltheadgt
• lttitlegtCS455 Introduction to Computer
  Networkslt/titlegt
• lt/headgt
• ltbodygt
• ltH1gtWelcome to Course CS455 - Introduction to
  Computer Networkslt/H1gt
• This course focuses on the network principles and
  protocols,
• and looks inside the communication networks.ltbrgt
• The list of picturesltbrgt
• ltulgt
• ltligtPicture 1 ltimg src"pic1.gif"gt
• ltligtPicture 2 ltimg src"pic2.gif"gt
• ltligtPicture 3 ltimg src"pic3.gif"gt
• lt/ulgt
• lt/bodygt
• lth3gtEnjoy the class!lt/h3gt
• lt/htmlgt

27
Simple HTML file
28
HTTP overview

• HTTP hypertext transfer protocol
• Webs application layer protocol
• client/server model
• client browser that requests, receives,
  displays Web objects
• server web server sends objects in response to
  requests
• HTTP 1.0 RFC 1945
• HTTP 1.1 RFC 2068

HTTP request
PC running Explorer
HTTP response
HTTP request
Server running Apache Web server
HTTP response
Mac running Navigator
29
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

• Protocols that maintain state are complex!
• past history (state) must be maintained
• if server/client crashes, their views of state
  may be inconsistent, must be reconciled

30
HTTP connections

• Nonpersistent HTTP
• At most one object is sent over a TCP connection.
• HTTP/1.0 uses nonpersistent HTTP

• Persistent HTTP
• Multiple objects can be sent over single TCP
  connection between client and server.
• HTTP/1.1 uses persistent connections in default
  mode

31
Nonpersistent HTTP
(assume containing text and references to 3
jpeg images)

• Suppose user enters URL
• http//www.vancouver.wsu.edu/fac/song
  /cs455/test.html

• 1a. HTTP client initiates TCP connection to HTTP
  server (process) at www.vancouver.wsu.edu on port
  80

1b. HTTP server at host www.vancouver.wsu.edu
waiting for TCP connection at port 80. accepts
connection, notifying client
2. HTTP client sends HTTP request message
(containing URL) into TCP connection socket.
Message indicates that client wants object
fac/song/cs455/test.htm
3. HTTP server receives request message, forms
response message containing requested object, and
sends message into its socket
time
32
Nonpersistent HTTP (cont.)
4. HTTP server closes TCP connection.

• 5. HTTP client receives response message
  containing html file, displays html. Parsing
  html file, finds 3 referenced jpeg objects

time
6. Steps 1-5 repeated for each of 3 jpeg objects
33
Persistent HTTP
(assume containing text and references to 3
jpeg images)

• Suppose user enters URL
• http//www.vancouver.wsu.edu/fac/song
  /cs455/test.html

• 1a. HTTP client initiates TCP connection to HTTP
  server (process) at www.vancouver.wsu.edu on port
  80

1b. HTTP server at host www.vancouver.wsu.edu
waiting for TCP connection at port 80. accepts
connection, notifying client
2. HTTP client sends HTTP request message
(containing URL) into TCP connection socket.
Message indicates that client wants object
fac/song/cs455/test.htm
3. HTTP server receives request message, forms
response message containing requested object, and
sends message into its socket
time
34
Persistent HTTP (cont.)

• 5. HTTP client receives response message
  containing html file, displays html. Parsing
  html file, finds 3 referenced jpeg objects

time
6. Steps 2-5 repeated for each of 3 jpeg objects,
and server responses accordingly
7. HTTP server closes TCP connection.
35
Response time modeling

• Definition of RTT round trip time, time to send
  a small packet to travel from client to server
  and back.
• Response time
• one RTT to initiate TCP connection
• one RTT for HTTP request and first few bytes of
  HTTP response to return
• file transmission time
• total 2RTTtransmit time

36
Persistent HTTP

• Nonpersistent HTTP issues
• requires 2 RTTs per object
• OS must work and allocate host resources for each
  TCP connection
• but 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 processes are sent over connection

• Persistent without pipelining
• client issues new request only when previous
  response has been received
• one RTT for each referenced object
• Persistent with pipelining
• default in HTTP/1.1
• client sends requests as soon as it encounters a
  referenced object
• as little as one RTT for all the referenced
  objects

37
HTTP request message

• two types of HTTP messages request, response
• HTTP request message
• ASCII (human-readable format)

request line (GET, POST, HEAD commands)
GET /somedir/page.html HTTP/1.1 Host
www.someschool.edu User-agent
Mozilla/4.0 Connection close Accept-languagefr
(extra carriage return, line feed)
header lines
Carriage return, line feed indicates end of
message
Connection keep-alive
38
HTTP request message general format
Entity Body used in POST method
39
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?monkeysbanana
40
Method types

• HTTP/1.0
• GET
• POST
• HEAD
• asks server to leave requested object out of
  response usually for debug purpose

• 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

41
HTTP response message
status line (protocol status code status phrase)
HTTP/1.1 200 OK Connection close Date Thu, 06
Aug 1998 120015 GMT Server Apache/1.3.0
(Unix) Last-Modified Mon, 22 Jun 1998 ...
Content-Length 6821 Content-Type text/html
data data data data data ...
header lines
data, e.g., requested HTML file
42
HTTP response status codes
In first line in server-gtclient 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

43
Trying out HTTP (client side) for yourself

• 1. Telnet to your favorite Web server

Opens TCP connection to port 80 (default HTTP
server port) at vancouver.wsu.edu. Anything typed
in sent to port 80 at vancouver.wsu.edu
telnet www.vancouver.wsu.edu 80
2. Type in a GET HTTP request
By typing this in (hit carriage return twice),
you send this minimal (but complete) GET request
to HTTP server
GET /fac/song/ HTTP/1.1 Host vancouver.wsu.edu
3. Look at response message sent by HTTP server!
44
User-server state cookies

• Many major Web sites use cookies
• Four components
• 1) cookie header line in the HTTP response
  message
• 2) cookie header line in HTTP request message
• 3) cookie file kept on users host and managed by
  users browser
• 4) back-end database at Web site

• Example
• Susan access Internet always from same PC
• She visits a specific e-commerce site for first
  time
• When initial HTTP requests arrives at site, site
  creates a unique ID and creates an entry in
  backend database for ID

45
Cookies keeping state (cont.)
(e.g., amazon)
server creates ID 1678 for user
entry in backend database
access
access
one week later
46
Cookies (continued)
aside

• Cookies and privacy
• cookies permit sites to learn a lot about you
• you may supply name and e-mail to sites
• search engines use redirection cookies to
  learn yet more
• advertising companies obtain info across sites

• What cookies can bring
• authorization
• shopping carts
• recommendations
• user session state (Web e-mail)

47
Web caches (proxy server)
Goal satisfy client request without involving
origin server

• browser sends all HTTP requests through cache
• object in cache cache returns object
• else cache requests object from origin server,
  then returns object to client

origin server
Proxy server
HTTP request
HTTP request
client
HTTP response
HTTP response
HTTP request
HTTP response
client
origin server
48
More about Web caching

• Cache acts as both client and server
• Typically cache is installed by ISP (university,
  company, residential ISP)

• Why Web caching?
• Reduce response time for client request.
• Reduce traffic on an institutions access link.
• Internet dense with caches enables poor content
  providers to effectively deliver content (so does
  P2P file sharing)

49
Caching example
origin servers

• Assumptions
• average object size 100,000 bits
• avg. request rate from institutions browsers to
  origin servers 15 reqs /sec
• Internet delay delay from the router A to any
  origin server and back to router, assume to be 2
  sec in average
• Access delay delay between router A and B
• LAN delay delay between browser and router B
• Consequences
• utilization on LAN (15100,000)/(10106) 15
• utilization on access link 100, access delay
  could be minutes
• total delay Internet delay access delay
  LAN delay
• 2 sec minutes milliseconds

public Internet
A
1.5 Mbps T1 access link
B
institutional network
10 Mbps LAN
institutional cache
50
Caching example (cont)
origin servers

• Possible solution
• increase bandwidth of access link to, say, 10
  Mbps
• Consequences
• utilization on LAN 15
• utilization on access link 15
• Total delay Internet delay access delay
  LAN delay
• 2 sec msecs msecs
• often a costly upgrade

public Internet
A
10 Mbps access link
B
institutional network
10 Mbps LAN
institutional cache
51
Caching example (cont)
origin servers

• Install cache
• suppose hit rate is 0.4
• Consequence
• 40 requests will be satisfied almost immediately
• 60 requests satisfied by origin server
• utilization of access link reduced to 60,
  resulting in negligible delays (say 10 msec)
  usually the delay on less than 80 utilization
  is much smaller than 100 unitilization
• total avg delay Internet delay access delay
  LAN delay .4(0.01)secs.6(2.01) secs
  milliseconds lt 1.4 secs

public Internet
A
1.5 Mbps access link
B
institutional network
10 Mbps LAN
institutional cache
52
Conditional GET
server
cache
HTTP request msg If-modified-since ltdategt

• Goal dont send object if cache has up-to-date
  cached version
• cache specify date of cached copy in HTTP
  request
• If-modified-since ltdategt
• server response contains no object if cached
  copy is up-to-date
• HTTP/1.0 304 Not Modified

object not modified
HTTP request msg If-modified-since ltdategt
object modified
HTTP response HTTP/1.0 200 OK ltdatagt
53
Hierarchical caching

• Hierarchical caching with three proxies.

54
Content Delivery Networks
Deployed by Content distribution companies, such
as Akamai

• (a) Original Web page. (b) Same page after
  transformation.

55
Content Delivery Networks

• Steps in looking up a URL when a CDN is used.

56
Application layer Roadmap

• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail
• SMTP, POP3, IMAP
• DNS
• P2P file sharing
• Multimedia network systems

57
FTP the file transfer protocol
file transfer
user at host
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

58
FTP separate control, data connections
TCP control connection port 21 (persistent)

• FTP client contacts FTP server at port 21,
  specifying TCP as transport protocol
• Client obtains authorization over control
  connection
• Client browses remote directory by sending
  commands over control connection.
• When server receives a command for a file
  transfer, the server opens a TCP data connection
  to client
• After transferring one file, server closes data
  connection.

TCP data connection port 20 (non-persistent)
FTP client
FTP server

• Server opens a second TCP data connection to
  transfer another file.
• Control connection out of band
• FTP server maintains state current directory,
  earlier authentication

59
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 Cant open data connection
• 452 Error writing file

60
Application layer Roadmap

• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail
• SMTP, POP3, IMAP
• DNS
• P2P file sharing
• Multimedia network systems

61
Scenario Alice sends message to Bob

• 5) Bobs mail server places the message in Bobs
  mailbox
• 6) POP3/IMAP Bobs mail server invokes his user
  agent (if online) to read message

• 1) Alice uses UA(user agent) to compose message
  to bob_at_someschool.edu
• 2) SMTP Alices UA sends message to her mail
  server message placed in message queue
• 3,4) SMTP Alices mail server opens SMTP
  connection with Bobs mail server and sends her
  message over the connection

1
2
6
3
4
5
62
Electronic Mail
SMTP

• Four major components
• user agents
• mail servers
• simple mail transfer protocol SMTP
• Mail access protocol POP3 or IMAP, etc
• User Agent
• composing, editing, sending, retrieving and
  reading mail messages
• e.g., Eudora, Outlook, elm, Netscape Messenger
• outgoing, incoming messages stored on server

POP3/IMAP
63
Electronic Mail mail servers
SMTP

• 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

POP3/IMAP
64
Electronic Mail SMTP RFC 2821

• uses TCP to reliably transfer email message from
  client to serverport 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

65
Sample SMTP interaction
S 220 wsu.edu C HELO iit.edu
S 250 Hello iit.edu, pleased to meet you
C MAIL FROM ltalice_at_iit.edugt S 250
alice_at_iit.edu... Sender ok C RCPT TO
ltbob_at_wsu.edugt S 250 bob_at_wsu.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 wsu.edu
closing connection
66
Try SMTP interaction for yourself

• telnet mail.vancouver.wsu.edu 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)
• Dangerous anyone could pretend to be you to send
  an email!

67
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 TCP connection initiated by receiver
• SMTP push TCP connection initiated by sender
• both have ASCII command/response interaction,
  status codes
• HTTP each object encapsulated in its own
  response msg
• SMTP multiple objects sent in one multipart msg

68
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 commands!
• body
• the message, ASCII characters only

header
blank line
body
69
Message format multimedia extensions

• MIME multimedia mail extension, RFC 2045, 2056
• additional lines in msg header declare MIME
  content type

MIME version
method used to encode data
multimedia data type, subtype, parameter
declaration
encoded data
70
Mail access protocols
SMTP
access protocol
receivers mail server

• SMTP delivery/storage to receivers server
  port 25
• No authorization dangerous!
• Mail access protocol retrieval from server
• POP3 Post Office Protocol RFC 1939 port 110
• authorization (agent lt--gtserver) and download
• IMAP Internet Mail Access Protocol RFC 1730
  port 143
• more features (more complex)
• manipulation of stored msgs on server
• HTTP Hotmail , Yahoo! Mail, etc.

71
POP3 protocol
S OK POP3 server ready C user bob S OK
C pass hungry S OK user successfully logged
on

• telnet mail.vancouver.wsu.edu 110
• 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

C list S 1 498 S 2 912
S . C retr 1 S ltmessage 1
contentsgt S . C dele 1 C retr
2 S ltmessage 1 contentsgt S .
C dele 2 C quit S OK POP3 server
signing off
72
POP3 (more) and IMAP

• More about POP3
• Previous example uses download and delete mode.
• Bob cannot re-read e-mail if he changes client
• Download-and-keep copies of messages on
  different clients
• POP3 is stateless across sessions

• IMAP
• Keep all messages in one place the server
• Allows user to organize messages in folders
• IMAP keeps user state across sessions
• names of folders and mappings between message IDs
  and folder name

73
Application layer Roadmap

• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail
• SMTP, POP3, IMAP
• DNS
• P2P file sharing
• Multimedia network systems

74
DNS Domain Name System

• DNS services
• Hostname to IP address translation
• Host aliasing
• Canonical and alias names
• Mail server aliasing
• Load distribution
• Replicated Web servers set of IP addresses for
  one canonical name

• Internet hosts, routers
• IP address (32 bit) - used for addressing
  datagrams
• name, e.g., www.wsu.edu - used by humans
• Q how to map between IP addresses and name ?

75
DNS Domain Name System

• Domain Name System
• distributed database implemented in hierarchy of
  many name servers
• application-layer protocol host, routers, name
  servers to communicate to resolve names
  (address/name translation)
• note core Internet function, implemented as
  application-layer protocol
• complexity at networks edge
• Why not centralize DNS?
• single point of failure
• traffic volume
• distant centralized database
• maintenance

76
Distributed, Hierarchical Database

• 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

77
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

13 root name servers worldwide
78
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, cn.
• Company Network Solutions maintains .com
• Educause maintains .edu
• Authoritative DNS servers organizations DNS
  servers, providing authoritative hostname to IP
  mappings for organizations servers (e.g., Web
  and mail).
• Can be maintained by organization or service
  provider

79
Local Name Server

• Does not strictly belong to hierarchy
• Think it as a DNS server close to you, which
  could be an authoritative DNS server
• Each ISP (residential ISP, company, university)
  has one.
• Also called default name server
• When a host makes a DNS query, query is sent to
  its local DNS server
• Acts as a proxy, forwards query into hierarchy.

80
Example
root DNS server

• Host at cs.iit.edu wants IP address for
  www.vancouver.wsu.edu

2
3
TLD DNS server
4
5

• recursive query
• Please search this name for me until you find
  it
• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask this server

6
7
1
8
authoritative DNS server dns.vancouver.wsu.edu
requesting host cs.iit.edu
www.vancouver.wsu.edu
81
Recursive queries only
root DNS server
2
3
6
7
TLD DNS server
4
5

• recursive query
• puts burden of name resolution on contacted name
  server
• heavy load?

1
8
authoritative DNS server dns.vancouver.wsu.edu
requesting host cs.iit.edu
www.vancouver.wsu.edu
82
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
• Or even a previous queried name-IP pairs
• update/notify mechanisms under design by IETF
• RFC 2136
• http//www.ietf.org/html.charters/dnsind-charter.h
  tml

83
DNS records

• DNS distributed db storing resource records (RR)

• TypeA
• name is hostname
• value is IP address
• TypeNS
• name is domain (e.g. foo.com)
• value is IP address of authoritative name server
  for this domain
• TypeCNAME
• name is alias name for some cannonical (the
  real) name
• www.ibm.com is really servereast.backup2.ibm.com
  
• value is cannonical name
• TypeMX
• value is name of mailserver associated with name

• ttl time-to-live (seconds)
• Class IN, e.g., Internet

84
Inserting records into DNS

• Example just created startup Network Star
• Register name networkstar.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 (name, ttl, class, type, value)
• (networkstar.com,86400,IN,NS,dns1.networkstar.com)
  
• (dns1.networkstar.com,86400,IN,A,212.212.212.1)
• Put in authoritative server (dns1.networkstar.com)
  Type A record for www.networkstar.com and Type
  MX record for networkstar.com
• (www.networkstar.com,86400,IN,A,212.212.212.10)
• (networkstar.com,86400,IN,MX,mail.networkstar.co
  m)
• (mail.networkstar.com,86400,IN,A,212.212.212.12)

More details refer to http//www.dns.net/dnsrd/rr.
html
85
Application layer Roadmap

• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail
• SMTP, POP3, IMAP
• DNS
• P2P file sharing
• Multimedia network systems

86
P2P centralized directory

• original Napster design
• 1) when peer connects, it informs central server
• IP address
• content
• 2) Alice queries for Tom and Jerry
• 3) Alice requests file from Bob

87
Gnutella protocol

• Query messagesent over existing TCPconnections
• peers forwardQuery message
• QueryHit sent over reversepath

File transfer HTTP
Query
QueryHit
Query
Query
QueryHit
Query
QueryHit
Scalability limited scopeflooding
Query
88
Exploiting heterogeneity KaZaA

• Each peer is either a group leader or assigned to
  a group leader.
• TCP connection between peer and its group leader.
• TCP connections between some pairs of group
  leaders.
• Group leader tracks the content in all its
  children.

89
KaZaA Querying

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

90
Node Lookup in Peer-to-Peer Networks
(K1,V1)
B
A
C
insert(K1,V1)
retrieve (K1)
91
De Bruijn Graph
001
011
111
000
101
010
100
110
Routing from x1x2xk to y1y2yk follows x1x2xk
? x2xk y1? x3xk y1 y2 ? ? y1y2yk Routing from
any node to another is at most k hops.
The content-addressable network D2B, Fraigniaud
and Gauron, PODC 2003 CBRBrain provide content
based routing service over Internet backbone,
Song and Li, ICCCN 2004
92
Application layer Roadmap

• Principles of network applications
• Web and HTTP
• FTP
• Electronic Mail
• SMTP, POP3, IMAP
• DNS
• P2P file sharing
• Multimedia network systems

93
Streaming Audio

• A straightforward way to implement clickable
  music on a Web page, but not practically used
  because need wait for downloading
• In practice, we use RealTime Streaming Protocol
  (RTSP) buffer and play

94
User Control of Streaming Media RTSP

• What it doesnt do
• does not define how audio/video is encapsulated
  for streaming over network
• does not restrict how streamed media is
  transported it can be transported over UDP or
  TCP
• does not specify how the media player buffers
  audio/video

• HTTP
• Does not target multimedia content
• No commands for fast forward, etc.
• RTSP RFC 2326
• Client-server application layer protocol.
• For user to control display rewind, fast
  forward, pause, resume, repositioning, etc

95
RTSP out of band control

• RTSP messages are also sent out-of-band
• RTSP control messages use different port numbers
  than the media stream out-of-band.
• Port 554
• The media stream is considered in-band.
• RTP (Real-time Transport Protocol)

• FTP uses an out-of-band control channel
• A file is transferred over one TCP connection.
• Control information (directory changes, file
  deletion, file renaming, etc.) is sent over a
  separate TCP connection.
• The out-of-band and in-band channels use
  different port numbers.

96
RTSP Operation
97
Streaming Audio
When packets carry alternate samples, the loss of
a packet reduces the temporal resolution rather
than creating a gap in time.
98
Streaming Audio

• The media player buffers input from the media
  server and plays from the buffer rather than
  directly from the network.

99
Internet Radio

• More chanllenging need faster audio compression
  method

100
Voice over IP H.323 from ITU

• The H323 architectural model for Internet
  telephony.

101
Voice over IP H.323 from ITU
H.245 negotiate the compression rate, bit rate
and etc for a session Q.931 establish/release
connection, dial/ring tone, etc H.225 channel
between terminal and gatekeeper, manages
Registration/Admission/Status RTP transfer voice
stream RTCP manage RTP stream G.7xx encoding
and decoding speech

• The H323 protocol stack.

102
Voice over IP H.323 from ITU

• Logical channels between the caller and callee
  during a call.

103
Voice over IP SIP from IETF

• Use a proxy and location servers with SIP
  (Session Initiation Protocol)
• SIP could be over TCP or UDP, and handles setup,
  management and terminate sessions

104
Setting up a call to a known IP address

• HTTP-style text messages
• Alices SIP invite message indicates her port
  number IP address. Indicates encoding that
  Alice prefers to receive (PCM ulaw)
• Bobs 200 OK message indicates his port number,
  IP address preferred encoding (GSM)
• SIP messages can be sent over TCP or UDP here
  sent over RTP/UDP.
• Default SIP port number is 5060.

105
Comparison of H.323 and SIP
106
Summary

• Our study of network apps now complete!

• specific protocols
• HTTP
• FTP
• SMTP, POP, IMAP
• DNS
• multimedia network system

• Application architectures
• client-server
• P2P
• hybrid
• application service requirements
• reliability, bandwidth, delay

107
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

• control vs. data msgs
• in-band, out-of-band
• centralized vs. decentralized
• stateless vs. stateful
• reliable vs. unreliable msg transfer
• complexity at network edge

108
Suggestions

• Read Chapter 2 (Application)
• Visit www.ietf.org to see whats going on
  new/improved protocols?
• Try ethereal lab http//www.ethereal.com/distribu
  tion/
• Review Chapter 3 (Transport)