Title: 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
2Unit 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
3Line 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)
4Line 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
5Transmission 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
6Transmission 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
7Transmission Characteristics of Guided Media
8Twisted 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
9Unshielded 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
10Near 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
11Coaxial Cable
12Optical 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
13Optical 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
14Cable 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
15Cable Modem Scheme
16UNIT 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.
17responsibilities of data link layer
- a) Framing
- b) Physical addressing
- c) Flow control
- d) Error control
- e) Access control
182.1 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
19Error Detection Process
204 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.
21Cyclic 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.
22Error Correction Process
23Flow 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
24Stop 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
25Stop and Wait Link Utilization
26Sliding 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
27Sliding Window Diagram
28Sliding Window Example
29Error 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
30Automatic Repeat Request (ARQ)
- collective name for such error control
mechanisms, including - stop and wait
- go back N
- selective reject (selective retransmission)
31Stop 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
32Stop and wait
- see example with both types of errors
- pros and cons
- simple
- inefficient
33Go 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
34Go 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
35Go 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
36Selective 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
37Go Back N vsSelective Reject
38High 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
39HDLC 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
40HDLC Frame Structure
- synchronous transmission of frames
- single frame format used
41Address 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
42Control 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
43Control 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
44Information 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
45HDLC 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
46Timers 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)
47Ethernet.
- 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
48UNIT III
- NETWORK LAYER 10
- Internetworks - Packet Switching and Datagram
approach IP addressing methods Subnetting
Routing Distance Vector Routing Link State
Routing Routers
49Packet 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
50Advantages
- 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
51Switching 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
52Virtual 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
53Routing 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
54Routing 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
55Distance 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
56Bridges
- Types of bridges
- Simple bridge
- Multiport bridge
- Transparent bridge
57Subnetting
- Three levels of hierarchy
- Masking
- Masks without subnetting
- Masks with subnetting
- Finding the subnetwork address
- Boundary level masking
- Non-boundary level masking
58UNIT 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.
59Duties 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
60Multiplexing
- 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
61Sockets
- 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
62User 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
63TCP
- 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
64TCP 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
65TCP Header
66TCP 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
67TCP 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)
68TCP 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
69TCP 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
70TCP 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
71Congestion 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
72Retransmission 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)
73Exponential 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
74Karns 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
75Window 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
76Window Management
77Fast 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
78Effects of Congestion
79Mechanisms for Congestion Control
80Backpressure
- 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
81Choke 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
82Implicit 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
83Explicit 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
84Integrated 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
85ISA 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
86ISA in Router
87ISA 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
88Token Bucket Scheme
89Queuing 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
90UNIT V
- APPLICATION LAYER 7
- Domain Name Space (DNS)
- SMTP
- FDP
- HTTP
- WWW
- Security
- Cryptography.
915. 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
92Domain Names
93DNS 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
94Resource Records (RRs)
95DNS Operation
96DNS 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
97Name 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
985.2 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
99Basic 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
101Mail 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
102SMTP 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
103SMTP 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
104SMTP 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
105SMTP 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
106SMTP 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)
107FTP
- Transfer a file from one system to another.
- TCP connections
- Basic model of FTP
1085.4 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)
109HTTP 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
110HTTP 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
111Intermediate HTTP Systems
112HTTP Messages
113HTTP 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
114HTTP General Header Fields
- Cache-Control
- Connection
- Data
- Forwarded
- Keep-Alive
- Mime-Version
- Pragma
- Upgrade
115Request 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
116Status 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
117Response Header Fields
- Location
- Proxy-Authentication
- Public
- Retry-After
- Server
- WWW-Authenticate
118Entity 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
119Entity 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
120WWW
- Hypertext Hypermedia
- Browser Architecture
- Categories of Web Documents
- HTML
- CGI
- Java
121Network Security
- Security Requirements
- confidentiality - protect data content/access
- integrity - protect data accuracy
- availability - ensure timely service
- authenticity - protect data origin
122Passive 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
123Active 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
124Requirements 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
125type 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
126Cryptography 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.