Recent posts in Computer Networks

Day Date Contents Slides Assignments
 1  Oct 15 Introduction to Computer Networks
Networks- links and nodes, 
Based on links- simplex, half duplex, full duplex
Based on connection- point to point, multipoint
Circuit switching- dedicated channel is required, not efficient
Packet switching- store and forward, congestion and loss of packets may occur
Unicast, multicast, broadcast
Time division multiplexing, Frequency division multiplexing, statistical multiplexing
Errors- bit error, burst error, packet loss, node/link error
Layering- abstraction, modularity (i)OSI model- 7 layers (ii)TCP/IP- 4 layers
Encapsulation of packets in the seven layers
 2  Oct 17 Ethernet - 802.3 
CSMA/CD- carrier sense - detect whether the node is active or not, multiple access
10 Mbps(normal) but maybe 100 Mbps
Physical properties
Coaxial cable- 500 m, optical fibre, twisted pair, transciever- checks cable is idle/active
Host can have more than one ID- in wired connection(only one ID) but in 
wireless(different ID)
Repeater after 500 m(Normal LAN not more than 4 repeaters(upto 2500m))
Access control
MAC-Media Access Control- implemented in network adapters
Frame format of Ethernet- Max. data size is 1500 Bytes
Multicast, unicast, specific
Transmitter algorithm- 1-persistent, p-persistent, non-persistent
Minimum size of packet should be atleast 512 bits.
Exponential backoff algorithm
Network security
Confidentiality- encrypt message, Traffic confidentiality- even traffic is not visible, 
Data integrity- change bits in data, Originality -replay attack, Timeliness
DNS attack- valid URL mapped to invalid ID, Authentication, Denial of service attack
Cryptographic blocks, encryption algorithm, block ciphers
Symmetric ciphers(secret key ciphers)- DES, 3DES, AES
Asymmetric ciphers(public key ciphers)- A pair of keys- private key and public key, 
single way communication(authentication)
RSA- 1024 bit key, prime factorisation is exponential
Authenticators- digital signature
Hash-like functions- MAC message authentication code
Keys- session(short time), predistributed- public key infrastructure
Application- PGP,SSH, Transport- TLS, SSL, Internet- IPSec
Wireless- 802.11, WPA- wifi protected access
Firewall- implemented in router or end-host, filter traffic using IP or port address
 3 Oct 26 WiFi- 802.11
spread spectrum- depends on how much frequency band is allocated to each user
Frequency hopping-each time slot different frequencies using random number 
generator algorithm
Direct sequence- XOR data with n bit random number
802.11 n- 600Mbps, 802.11 b- direct sequence speed is 11 Mbps, 
802.11 a- OFDM, 54 Mbps, 5GHz, 802.11 g- 54 Mbps, 2.5 GHz
CSMA/CA - hidden node, exposed node, RTS-CTS signals
Access point- scanning- active and passive, probe frame, probe response 
Frame format(WiFi)
Internetworking- switches, bridges, forwarding
Connectionless-datagram, Connection-oriented- Virtual circuit- 4 tuple
Datagram delivery- best effort, no guarantee that receiver may recieve
IPv4 packet frame- fragmentation
IP Address- unique for all host, heirarchical, Class A, Class B, Class C
Subnetting- <subnet number, subnet mask, next hop>
subnet mask + IP Address = Subnet Number
Routing- (distributed)
Distance vector- count to infinity, RIP
Link State- Dijkstra, reliable flooding, system update
<neighbour, cost, nexthop, seq no, TTL>, OSPF
 4 Oct 28 Application layer protocols
SMTP- push, port-25,secure port-465
POP3- pull, 110, secure port 995
FTP- control(21) and data(20)
HTTP- www request-response service
DNS- hostname to IP, heirarchical, distributed database on top of named nodes
IPv4- 32 bit addresses separated by dots, frame format
IPv6- 128 bit hexadecimal rep separated by colons
Additional features- auto config, QoS, privacy security authentication, multicast routing
Sockets- process to process, socket descriptor, socket binding, types- datagram, stream, raw
UDP- unreliable, realtime communication
Frame format- psuedoheader, mapper service port
TCP- reliable, full duplex, inorder communication
flow control, congestion control, frame format- flags
End-to-end connectivity issues- establishment/teardown, timeout, out of order packets, resources required
Advertised window, effective window
Congestion control- 
AIMD- additive increment, multiplicative decrement, congestion window incremented by 1,
Slow start- exponential increase, 
Fast retransmit/fast recovery- retransmit after 3 failed acks and recovery
Error control- 2D parity- even parity , odd parity; Checksum- adding 16 bits of data using 1's complement, CRC
 5 Oct 29 CRC- how generator polynomial is used in calculation in both sender and reciever
Flow control- ARQ- automatic repeat request- timeout, Ack
Stop and wait
Four scenarios-(i) in simple one only after Ack next packet is send 
(ii)If a packet is lost it is resend after timeout 
(iii)If Ack is lost the packet is resend 
(iv) If an earlier Ack arrives after timeout still the packet is resend- duplicate packets in receiver
Utilisation is 50%, data packet size is around 1 KB
Sliding window- sender and receiver buffer and window sizes,synchronisation 
and out of order delivery in receiver
Two cases: i) Receiver window size is 1 - Go Back N- if a packet is lost the receiver will not 
receive any other packets and all the packets from the sequence number of lost packet
have to be retransmitted
ii) Receiver window size is equal to sender window size- Selective repeat
only the lost packet has to be retransmitted and after receiving the packet, the highest seq 
number packet is acknowledged.
Sequence number selection- problems
Sender window size less than (max seq num + 1)/2


Manoja Rajalakshmi A posted in Computer Networks Oct 29, 2018
Consider a TCP connection with 10 ms round-trip time, max segment size = 2 KB, receiver window = 64 KB. Suppose packet #4 timed out and all other transmissions were successful. RTO value is 50 ms. Calculate the time required to reach the receiver window size. Assume RTT is fixed.
Sandeep Verma posted in Computer Networks Aug 25, 2016
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