Department of Electronics

1
Department of Electronics communication
COMPUTER NETWORKS(CS1302) by A.Asha

• AIM
• To introduce the concept ,terminologies and
  technologies used in modern data communication
  and computer networking.
• OBJECTIVES
• To introduce the students the functions of
  different layers.
• To introduce IEEE standard employed in computer
  networking.
• To make students to get familiarized with
  different protocols and network components

2
Unit I

• DATA COMMUNICATIONS 8
• Components Direction of Data flow networks
  Components and Categories types of Connections
  Topologies Protocols and Standards ISO / OSI
  model Transmission Media Coaxial Cable
  Fiber Optics Line Coding Modems RS232
  Interfacing sequences

3
Line Configuration - Topology

• physical arrangement of stations on medium
• point to point - two stations
• such as between two routers / computers
• multi point - multiple stations
• traditionally mainframe computer and terminals
• now typically a local area network (LAN)

4
Line Configuration - Duplex

• simplex
• one direction eg. television
• half duplex (two-way alternate)
• only one station may transmit at a time
• requires one data path
• full duplex (two-way simultaneous)
• simultaneous transmission and reception between
  two stations
• requires two data paths
• separate media or frequencies used for each
  direction or echo canceling

5
Transmission Terminology

• data transmission occurs between a transmitter
  receiver via some medium
• guided medium
• eg. twisted pair, coaxial cable, optical fiber
• unguided / wireless medium
• eg. air, water, vacuum

6
Transmission Media- Overview

• guided - wire / optical fibre
• unguided - wireless
• characteristics and quality determined by medium
  and signal
• in unguided media - bandwidth produced by the
  antenna is more important
• in guided media - medium is more important
• key concerns are data rate and distance

7
Transmission Characteristics of Guided Media
8
Twisted Pair - Transmission Characteristics

• analog
• needs amplifiers every 5km to 6km
• digital
• can use either analog or digital signals
• needs a repeater every 2-3km
• limited distance
• limited bandwidth (1MHz)
• limited data rate (100MHz)
• susceptible to interference and noise

9
Unshielded vs Shielded

• unshielded Twisted Pair (UTP)
• ordinary telephone wire
• cheapest
• easiest to install
• suffers from external EM interference
• shielded Twisted Pair (STP)
• metal braid or sheathing that reduces
  interference
• more expensive
• harder to handle (thick, heavy)
• in a variety of categories - see EIA-568

10
Near End Crosstalk

• coupling of signal from one pair to another
• occurs when transmit signal entering the link
  couples back to receiving pair
• ie. near transmitted signal is picked up by near
  receiving pair

11
Coaxial Cable
12
Optical Fiber - Benefits

• greater capacity
• data rates of hundreds of Gbps
• smaller size weight
• lower attenuation
• electromagnetic isolation
• greater repeater spacing
• 10s of km at least

13
Optical Fiber - Transmission Characteristics

• uses total internal reflection to transmit light
• effectively acts as wave guide for 1014 to 1015
  Hz
• can use several different light sources
• Light Emitting Diode (LED)
• cheaper, wider operating temp range, lasts longer
• Injection Laser Diode (ILD)
• more efficient, has greater data rate
• relation of wavelength, type data rate

14
Cable Modems

• dedicate two cable TV channels to data transfer
• each channel shared by number of subscribers,
  using statistical TDM
• Downstream
• cable scheduler delivers data in small packets
• active subscribers share downstream capacity
• also allocates upstream time slots to subscribers
• Upstream
• user requests timeslots on shared upstream
  channel
• Headend scheduler notifies subscriber of slots to
  use

15
Cable Modem Scheme
16
UNIT II

• DATA LINK LAYER 12
• Error detection and correction Parity LRC
  CRC Hamming code Flow Control and Error
  control stop and wait go back N ARQ
  selective repeat ARQ- sliding window techniques
  HDLC.
• LAN Ethernet IEEE 802.3, IEEE 802.4, and IEEE
  802.5 IEEE 802.11FDDI, SONET Bridges.

17
responsibilities of data link layer

• a) Framing
• b) Physical addressing
• c) Flow control
• d) Error control
• e) Access control

18
2.1 Error detection and correction

• 2 types of errors
• a) Single-bit error.
• b) Burst-bit error.
• parity
• parity bit set so character has even (even
  parity) or odd (odd parity) number of ones
• even number of bit errors goes undetected

19
Error Detection Process
20
4 types of redundancy checks

• a) Vertical redundancy checks (VRC). The most
  common and least expensive mechanism for error
  detection is the vertical
• redundancy check (VRC) often called a
  parity check. In this technique a redundant
  bit 3 called a parity bit, is appended to every
  data unit so, that the total number of 0s in the
  unit (including the parity bit) becomes even.
• b) Longitudinal redundancy checks (LRC). In
  longitudinal redundancy check (LRC), a block
  of bits is divided into rows and a
• redundant row of bits is added to the whole
  block.
• c) Cyclic redundancy checks (CRC). A CRC checker
  functions exactly like a generator. After
  receiving the data appended with the CRC it
  does the same modulo-2 division. If the
  remainder is all 0s the CRC is dropped and
  the data accepted. Otherwise, the received stream
  of bits is discarded and the dates are resent.
• d) Checksum. The error detection method used by
  the higher layer protocol is called checksum.
  Checksum is based on the concept of redundancy.

21
Cyclic Redundancy Check

• one of most common and powerful checks
• The sender follows these steps
• a) The units are divided into k sections each of
  n bits.
• b) All sections are added together using 2s
  complement to get the sum.
• c) The sum is complemented and become the
  checksum.
• d) The checksum is sent with the data.

22
Error Correction Process
23
Flow Control

• ensure sending entity does not overwhelm
  receiving entity
• by preventing buffer overflow
• influenced by
• transmission time
• time taken to emit all bits into medium
• propagation time
• time for a bit to traverse the link
• assume here no errors but varying delays

24
Stop and Wait

• source transmits frame
• destination receives frame and replies with
  acknowledgement (ACK)
• source waits for ACK before sending next
• destination can stop flow by not send ACK
• works well for a few large frames
• Stop and wait becomes inadequate if large block
  of data is split into small frames

25
Stop and Wait Link Utilization
26
Sliding Windows Flow Control

• allows multiple numbered frames to be in transit
• receiver has buffer W long
• transmitter sends up to W frames without ACK
• ACK includes number of next frame expected
• sequence number is bounded by size of field (k)
• frames are numbered modulo 2k
• giving max window size of up to 2k - 1
• receiver can ack frames without permitting
  further transmission (Receive Not Ready)
• must send a normal acknowledge to resume
• if have full-duplex link, can piggyback ACks

27
Sliding Window Diagram
28
Sliding Window Example
29
Error Control

• detection and correction of errors such as
• lost frames
• damaged frames
• common techniques use
• error detection
• positive acknowledgment
• retransmission after timeout
• negative acknowledgement retransmission

30
Automatic Repeat Request (ARQ)

• collective name for such error control
  mechanisms, including
• stop and wait
• go back N
• selective reject (selective retransmission)

31
Stop and Wait

• source transmits single frame
• wait for ACK
• if received frame damaged, discard it
• transmitter has timeout
• if no ACK within timeout, retransmit
• if ACK damaged,transmitter will not recognize it
• transmitter will retransmit
• receive gets two copies of frame
• use alternate numbering and ACK0 / ACK1

32
Stop and wait

• see example with both types of errors
• pros and cons
• simple
• inefficient

33
Go Back N

• based on sliding window
• if no error, ACK as usual
• use window to control number of outstanding
  frames
• if error, reply with rejection
• discard that frame and all future frames until
  error frame received correctly
• transmitter must go back and retransmit that
  frame and all subsequent frames

34
Go Back N - Handling

• Damaged Frame
• error in frame i so receiver rejects frame i
• transmitter retransmits frames from i
• Lost Frame
• frame i lost and either
• transmitter sends i1 and receiver gets frame i1
  out of seq and rejects frame i
• or transmitter times out and send ACK with P bit
  set which receiver responds to with ACK i
• transmitter then retransmits frames from i

35
Go Back N - Handling

• Damaged Acknowledgement
• receiver gets frame i, sends ack (i1) which is
  lost
• acks are cumulative, so next ack (in) may arrive
  before transmitter times out on frame i
• if transmitter times out, it sends ack with P bit
  set
• can be repeated a number of times before a reset
  procedure is initiated
• Damaged Rejection
• reject for damaged frame is lost
• handled as for lost frame when transmitter times
  out

36
Selective Reject

• also called selective retransmission
• only rejected frames are retransmitted
• subsequent frames are accepted by the receiver
  and buffered
• minimizes retransmission
• receiver must maintain large enough buffer
• more complex logic in transmitter
• hence less widely used
• useful for satellite links with long propagation
  delays

37
Go Back N vsSelective Reject
38
High Level Data Link Control (HDLC)

• an important data link control protocol
• specified as ISO 33009, ISO 4335
• station types
• Primary - controls operation of link
• Secondary - under control of primary station
• Combined - issues commands and responses
• link configurations
• Unbalanced - 1 primary, multiple secondary
• Balanced - 2 combined stations

39
HDLC Transfer Modes

• Normal Response Mode (NRM)
• unbalanced config, primary initiates transfer
• used on multi-drop lines, eg host terminals
• Asynchronous Balanced Mode (ABM)
• balanced config, either station initiates
  transmission, has no polling overhead, widely
  used
• Asynchronous Response Mode (ARM)
• unbalanced config, secondary may initiate
  transmit without permission from primary, rarely
  used

40
HDLC Frame Structure

• synchronous transmission of frames
• single frame format used

41
Address Field

• identifies secondary station that sent or will
  receive frame
• usually 8 bits long
• may be extended to multiples of 7 bits
• LSB indicates if is the last octet (1) or not (0)
• all ones address 11111111 is broadcast

42
Control Field

• different for different frame type
• Information - data transmitted to user (next
  layer up)
• Flow and error control piggybacked on information
  frames
• Supervisory - ARQ when piggyback not used
• Unnumbered - supplementary link control
• first 1-2 bits of control field identify frame
  type

43
Control Field

• use of Poll/Final bit depends on context
• in command frame is P bit set to1 to solicit
  (poll) response from peer
• in response frame is F bit set to 1 to indicate
  response to soliciting command
• seq number usually 3 bits
• can extend to 8 bits as shown below

44
Information FCS Fields

• Information Field
• in information and some unnumbered frames
• must contain integral number of octets
• variable length
• Frame Check Sequence Field (FCS)
• used for error detection
• either 16 bit CRC or 32 bit CRC

45
HDLC Operation

• consists of exchange of information, supervisory
  and unnumbered frames
• have three phases
• initialization
• by either side, set mode seq
• data transfer
• with flow and error control
• using both I S-frames (RR, RNR, REJ, SREJ)
• disconnect
• when ready or fault noted

46
Timers and time registers in FDDI.

• Time registers
• Synchronous allocation(SA)
• Target token rotation time(TTRT)
• Absolute maximum time(AMT)
• Timers
• Token rotation timer(TRT)
• Token holding timer(THT)

47
Ethernet.

• Access method CSMA/CD
• Addressing
• Electrical specification
• Frame format
• Implementation
• 10 base 5 Thick Ethernet
• 10 base 2 Thin Ethernet
• 10 base T Twisted-pair Ethernet
• 1 base 5 Star LAN

48
UNIT III

• NETWORK LAYER 10
• Internetworks - Packet Switching and Datagram
  approach IP addressing methods Subnetting
  Routing Distance Vector Routing Link State
  Routing Routers

49
Packet Switching

• circuit switching was designed for voice
• packet switching was designed for data
• transmitted in small packets
• packets contains user data and control info
• user data may be part of a larger message
• control info includes routing (addressing) info
• packets are received, stored briefly (buffered)
  and past on to the next node

50
Advantages

• line efficiency
• single link shared by many packets over time
• packets queued and transmitted as fast as
  possible
• data rate conversion
• stations connects to local node at own speed
• nodes buffer data if required to equalize rates
• packets accepted even when network is busy
• priorities can be used

51
Switching Techniques

• Datagram approach
• Virtual circuit approach
• Switched virtual circuit(SVC)
• Permanent virtual circuit(PVC)
• Circuit switched connection versus virtual
  circuit connection
• Path versus route
• Dedicated versus shared

52
Virtual Circuits v Datagram

• virtual circuits
• network can provide sequencing and error control
• packets are forwarded more quickly
• less reliable
• datagram
• no call setup phase
• more flexible
• more reliable

53
Routing in Packet Switched Network

• key design issue for (packet) switched networks
• select route across network between end nodes
• characteristics required
• correctness
• simplicity
• robustness
• stability
• fairness
• optimality
• efficiency

54
Routing Strategies - Fixed Routing

• use a single permanent route for each source to
  destination pair
• determined using a least cost algorithm
• route is fixed
• at least until a change in network topology
• hence cannot respond to traffic changes
• advantage is simplicity
• disadvantage is lack of flexibility

55
Distance vector routing and link state routing.

• Distance vector routing
• Sharing information
• Routing table
• Creating the table
• Updating the table
• Updating algorithm
• Link state routing
• Information sharing
• Packet cost
• Link state packet
• Getting information about neighbors
• Initialization
• Link state database

56
Bridges

• Types of bridges
• Simple bridge
• Multiport bridge
• Transparent bridge

57
Subnetting

• Three levels of hierarchy
• Masking
• Masks without subnetting
• Masks with subnetting
• Finding the subnetwork address
• Boundary level masking
• Non-boundary level masking

58
UNIT IV

• TRANSPORT LAYER 8
• Duties of transport layer Multiplexing
  Demultiplexing Sockets User Datagram Protocol
  (UDP) Transmission Control Protocol (TCP)
  Congestion Control Quality of services (QOS)
  Integrated Services.

59
Duties of transport layer

• end-to-end data transfer service
• shield upper layers from network details
• reliable, connection oriented
• has greater complexity
• eg. TCP
• best effort, connectionless
• datagram
• eg. UDP

60
Multiplexing

• of upper layers (downward multiplexing)
• so multiple users employ same transport protocol
• user identified by port number or service access
  point
• may also multiplex with respect to network
  services used (upward multiplexing)
• eg. multiplexing a single virtual X.25 circuit to
  a number of transport service user

61
Sockets

• process sends/receives messages to/from its
  socket
• ?? socket analogous to mailbox
• ?? sending process relies on transport
  infrastructure which brings message to socket at
  receiving process

62
User Datagram Protocol(UDP)

• connectionless service for application level
  procedures specified in RFC 768
• unreliable
• delivery duplication control not guaranteed
• reduced overhead
• least common denominator service
• uses
• inward data collection
• outward data dissemination
• request-response
• real time application

63
TCP

• Transmission Control Protocol (RFC 793)
• connection oriented, reliable communication
• over reliable and unreliable (inter)networks
• two ways of labeling data
• data stream push
• user requires transmission of all data up to push
  flag
• receiver will deliver in same manner
• avoids waiting for full buffers
• urgent data signal
• indicates urgent data is upcoming in stream
• user decides how to handle it

64
TCP Services

• a complex set of primitives
• incl. passive active open, active open with
  data, send, allocate, close, abort, status
• passive open indicates will accept connections
• active open with data sends data with open
• and parameters
• incl. source port, destination port address,
  timeout, security, data, data length, PUSH
  URGENT flags, send receive windows, connection
  state, amount awaiting ACK

65
TCP Header
66
TCP and IP

• not all parameters used by TCP are in its header
• TCP passes some parameters down to IP
• precedence
• normal delay/low delay
• normal throughput/high throughput
• normal reliability/high reliability
• security
• min overhead for each PDU is 40 octets

67
TCP Mechanisms Connection Establishment

• three way handshake
• SYN, SYN-ACK, ACK
• connection determined by source and destination
  sockets (host, port)
• can only have a single connection between any
  unique pairs of ports
• but one port can connect to multiple different
  destinations (different ports)

68
TCP Mechanisms Data Transfer

• data transfer a logical stream of octets
• octets numbered modulo 223
• flow control uses credit allocation of number of
  octets
• data buffered at transmitter and receiver
• sent when transport entity ready
• unless PUSH flag used to force send
• can flag data as URGENT, sent immediately
• if receive data not for current connection, RST
  flag is set on next segment to reset connection

69
TCP Mechanisms Connection Termination

• graceful close
• TCP user issues CLOSE primitive
• transport entity sets FIN flag on last segment
  sent with last of data
• abrupt termination by ABORT primitive
• entity abandons all attempts to send or receive
  data
• RST segment transmitted to other end

70
TCP Implementation Options

• TCP standard precisely specifies protocol
• have some implementation policy options
• send
• deliver
• accept
• retransmit
• acknowledge
• implementations may choose alternative options
  which may impact performance

71
Congestion Control

• flow control also used for congestion control
• recognize increased transit times dropped
  packets
• react by reducing flow of data
• RFCs 1122 2581 detail extensions
• Tahoe, Reno NewReno implementations
• two categories of extensions
• retransmission timer management
• window management

72
Retransmission Timer Management

• static timer likely too long or too short
• estimate round trip delay by observing pattern of
  delay for recent segments
• set time to value a bit greater than estimate
• simple average over a number of segments
• exponential average using time series (RFC793)
• RTT Variance Estimation (Jacobsons algorithm)

73
Exponential RTO Backoff

• timeout probably due to congestion
• dropped packet or long round trip time
• hence maintaining RTO is not good idea
• better to increase RTO each time a segment is
  re-transmitted
• RTO qRTO
• commonly q2 (binary exponential backoff)
• as in ethernet CSMA/CD

74
Karns Algorithm

• if segment is re-transmitted, ACK may be for
• first copy of the segment (longer RTT than
  expected)
• second copy
• no way to tell
• dont measure RTT for re-transmitted segments
• calculate backoff when re-transmission occurs
• use backoff RTO until ACK arrives for segment
  that has not been re-transmitted

75
Window Management

• slow start
• larger windows cause problem on connection
  created
• at start limit TCP to 1 segment
• increase when data ACK, exponential growth
• dynamic windows sizing on congestion
• when a timeout occurs perhaps due to congestion
• set slow start threshold to half current
  congestion window
• set window to 1 and slow start until threshold
• beyond threshold, increase window by 1 for each
  RTT

76
Window Management
77
Fast Retransmit Fast Recovery

• retransmit timer rather longer than RTT
• if segment lost TCP slow to retransmit
• fast retransmit
• if receive 4 ACKs for same segment then
  immediately retransmit since likely lost
• fast recovery
• lost segment means some congestion
• halve window then increase linearly
• avoids slow-start

78
Effects of Congestion
79
Mechanisms for Congestion Control
80
Backpressure

• if node becomes congested it can slow down or
  halt flow of packets from other nodes
• cf. backpressure in blocked fluid pipe
• may mean that other nodes have to apply control
  on incoming packet rates
• propagates back to source
• can restrict to high traffic logical connections
• used in connection oriented nets that allow hop
  by hop congestion control (eg. X.25)
• not used in ATM nor frame relay
• only recently developed for IP

81
Choke Packet

• a control packet
• generated at congested node
• sent to source node
• eg. ICMP source quench
• from router or destination
• source cuts back until no more source quench
  message
• sent for every discarded packet, or anticipated
• is a rather crude mechanism

82
Implicit Congestion Signaling

• transmission delay increases with congestion
• hence a packet may be discarded
• source detects this implicit congestion
  indication
• useful on connectionless (datagram) networks
• eg. IP based
• (TCP includes congestion and flow control - see
  chapter 17)
• used in frame relay LAPF

83
Explicit Congestion Signaling

• network alerts end systems of increasing
  congestion
• end systems take steps to reduce offered load
• Backwards
• congestion avoidance notification in opposite
  direction to packet required
• Forwards
• congestion avoidance notification in same
  direction as packet required

84
Integrated Services

• changes in traffic demands require variety of
  quality of service
• eg. internet phone, multimedia, multicast
• new functionality required in routers
• new means of requesting QoS
• IETF developing a suite of Integrated Services
  Architecture (ISA) standards
• RFC 1633 defines overall view of ISA

85
ISA Approach

• IP nets control congestion by
• routing algorithms
• packet discard
• ISA provides enhancements to traditional IP
• in ISA associate each packet with a flow
• ISA functions
• admission control
• routing algorithm
• queuing discipline
• discard policy

86
ISA in Router
87
ISA Services

• Guaranteed
• assured data rate
• upper bound on queuing delay
• no queuing loss
• Controlled load
• approximates best effort behavior on unloaded net
• no specific upper bound on queuing delay
• very high delivery success
• Best Effort
• traditional IP service

88
Token Bucket Scheme
89
Queuing Discipline

• traditionally FIFO
• no special treatment for high priority flow
  packets
• large packet can hold up smaller packets
• greedy connection can crowd out less greedy
  connection
• need some form of fair queuing
• multiple queues used on each output port
• packet is placed in queue for its flow
• round robin servicing of queues
• can have weighted fair queuing

90
UNIT V

• APPLICATION LAYER 7
• Domain Name Space (DNS)
• SMTP
• FDP
• HTTP
• WWW
• Security
• Cryptography.

91
5. 1 DNSThe Internet Directory Service

• the Domain Name Service (DNS) provides mapping
  between host name IP address
• defined in RFCs 1034 / 1035
• key elements
• domain name space
• DNS database
• name servers
• name resolvers

92
Domain Names
93
DNS Database

• hierarchical database
• containing resource records (RRs)
• features
• variable-depth hierarchy for names
• distributed database
• distribution controlled by database
• provides name-to-address directory service for
  network applications

94
Resource Records (RRs)
95
DNS Operation
96
DNS Server Hierarchy

• DNS database is distributed hierarchically
• may extend as deep as needed
• any organization owning a domain can run name
  servers
• each server manages authoritative name data for a
  zone
• 13 root name servers at top of hierarchy share
  responsibility for top level zones

97
Name Resolution

• query begins with name resolver on host
• knows name/address of local DNS server
• given a name request, the resolver can
• return name from cache if already known
• send DNS query to local server which may return
  answer, or query other servers
• recursive technique - server queries other
  servers for resolver
• iterative technique - resolver queries servers in
  turn as needed

98
5.2 SMTP

• RFC 821
• not concerned with format of messages or data
• covered in RFC 822 (see later)
• SMTP uses info written on envelope of mail
• message header
• does not look at contents
• message body
• except
• standardize message character set to 7 bit ASCII
• add log info to start of message

99
Basic Operation

• email message is created by user agent program
  (mail client), and consists of
• header with recipients address and other info
• body containing user data
• messages queued and sent as input to SMTP sender
  program
• yypically a server process (daemon on UNIX)

100
SMTP Mail Flow
101
Mail Message Contents

• each queued message has two parts
• message text
• RFC 822 header with envelope and list of
  recipients
• message body, composed by user
• list of mail destinations
• derived by user agent from header
• may be listed in header
• may require expansion of mailing lists
• may need replacement of mnemonic names with
  mailbox names
• if BCCs indicated, user agent needs to prepare
  correct message format

102
SMTP Sender

• takes message from queue
• transmits to proper destination host
• via SMTP transaction
• over one or more TCP connections to port 25
• host may have multiple senders active
• host must create receivers on demand
• when delivery complete, sender deletes
  destination from list for that message
• when all destinations processed, message is
  deleted

103
SMTP Protocol - Reliability

• used to transfer messages from sender to receiver
  over TCP connection
• attempts to provide reliable service
• no guarantee to recover lost messages
• no end to end acknowledgement to originator
• error indication delivery not guaranteed
• generally considered reliable

104
SMTP Receiver

• accepts arriving message
• places in user mailbox or copies to outgoing
  queue for forwarding
• receiver must
• verify local mail destinations
• deal with errors
• sender responsible for message until receiver
  confirm complete transfer
• indicates mail has arrived at host, not user

105
SMTP Forwarding

• mostly direct transfer from sender host to
  receiver host
• may go through intermediate machine via
  forwarding capability
• sender can specify route
• target user may have moved

106
SMTP Replies

• positive completion reply (2xx)
• e.g. 220 ltdomaingt Service ready
• e.g. 250 Requested mail action okay, completed
• positive intermediate reply (3xx)
• e.g. 354 Start mail input end with ltCRLFgt.ltCRLFgt
• transient negative completion reply (4xx)
• e.g. 452 Requested action not taken insufficient
  system storage
• permanent negative completion reply (5xx)
• e.g. 500 Syntax error, command unrecognized
• e.g. 550 Requested action not taken mailbox
  unavailable (e.g., mailbox not found, no access)

107
FTP

• Transfer a file from one system to another.
• TCP connections
• Basic model of FTP

108
5.4 Hypertext Transfer ProtocolHTTP

• base protocol for World Wide Web
• for any hypertext client/server application
• is a protocol for efficiently transmitting
  information to make hypertext jumps
• can transfer plain text, hypertext, audio,
  images, and Internet accessible information
• versions 0.9, 1.0, now 1.1 (RFC2616)

109
HTTP Overview

• transaction oriented client/server protocol
• between Web browser (client) and Web server
• uses TCP connections
• stateless
• each transaction treated independently
• each new TCP connection for each transaction
• terminate connection when transaction complete
• flexible format handling
• client may specify supported formats

110
HTTP Operation - Caches

• often have a web cache
• stores previous requests/ responses
• may return stored response to subsequent requests
• may be a client, server or intermediary system
• not all requests can be cached

111
Intermediate HTTP Systems
112
HTTP Messages
113
HTTP Messages BNF Format

• HTTP-Message Simple-Request Simple-Response
  Full-Request Full-Response
• Full-Request Request-Line
• ( General-Header Request-Header
  Entity-Header )
• CRLF
• Entity-Body
• Full-Response Status-Line
• ( General-Header Response-Header
  Entity-Header )
• CRLF
• Entity-Body
• Simple-Request "GET" SP Request-URL CRLF
• Simple-Response Entity-Body

114
HTTP General Header Fields

• Cache-Control
• Connection
• Data
• Forwarded
• Keep-Alive
• Mime-Version
• Pragma
• Upgrade

115
Request Methods

• request-line has
• method
• Request URL
• HTTP version
• Request-Line Method Request-URL HTTP-Version
  CRLF
• HTTP/1.1 methods
• OPTIONS, GET, HEAD, POST, PUT, PATCH, COPY, MOVE,
  DELETE, LINK, UNLINK, TRACE, WRAPPED,
  Extension-method

116
Status Codes

• informational - headers only
• successful - headers body if relevant
• redirection - further action needed
• client error - has syntax or other error
• server error - failed to satisfy valid request

117
Response Header Fields

• Location
• Proxy-Authentication
• Public
• Retry-After
• Server
• WWW-Authenticate

118
Entity Header Fields

• Allow
• Content-Encoding
• Content-Language
• Content-Length
• Content-MD5
• Content-Range
• Content-Type
• Content-Version
• Derived-From

• Expires
• Last-Modified
• Link
• Title
• Transfer-Encoding
• URL-Header
• Extension-Header

119
Entity Body

• entity body is an arbitrary sequence of octets
• HTTP can transfer any type of data including
• text, binary data, audio, images, video
• data is content of resource identified by URL
• interpretation data determined by header fields
• Content-Type - defines data interpretation
• Content-Encoding - applied to data
• Transfer-Encoding - used to form entity body

120
WWW

• Hypertext Hypermedia
• Browser Architecture
• Categories of Web Documents
• HTML
• CGI
• Java

121
Network Security

• Security Requirements
• confidentiality - protect data content/access
• integrity - protect data accuracy
• availability - ensure timely service
• authenticity - protect data origin

122
Passive Attacks

• eavesdropping on transmissions
• to obtain information
• release of possibly sensitive/confidential
  message contents
• traffic analysis which monitors frequency and
  length of messages to get info on senders
• difficult to detect
• can be prevented using encryption

123
Active Attacks

• masquerade
• pretending to be a different entity
• replay
• modification of messages
• denial of service
• easy to detect
• detection may lead to deterrent
• hard to prevent
• focus on detection and recovery

124
Requirements for Security

• strong encryption algorithm
• even known, unable to decrypt without key
• even if many plaintexts ciphertexts available
• sender and receiver must obtain secret key
  securely
• once key is known, all communication using this
  key is readable

125
type of encryption/decryption method

• Conventional Methods
• Character-Level Encryption Substitutional
  Transpositional
• Bit-Level Encryption Encoding/Decoding,
  Permutation, Substitution, Product,
• Exclusive-Or Rotation
• Public key Methods

126
Cryptography RSA Security

• brute force search of all keys
• given size of parameters is infeasible
• but larger keys do slow calculations
• factor n to recover p q
• a hard problem
• well known 129 digit challenge broken in 1994
• key size of 1024-bits (300 digits) currently
  secure for most apps

127

• TEXT BOOKS
• Behrouz A. Foruzan, Data communication and
  Networking, Tata McGraw-Hill, 2004.
• REFERENCES
• James .F. Kurouse W. Rouse, Computer
  Networking A Topdown Approach Featuring,
  Pearson Education.
• Larry L.Peterson Peter S. Davie, COMPUTER
  NETWORKS, Harcourt Asia Pvt. Ltd., Second
  Edition.
• Andrew S. Tannenbaum, Computer Networks, PHI,
  Fourth Edition, 2003.
• William Stallings, Data and Computer
  Communication, Sixth Edition, Pearson Education,
  2000.