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Computer Networks

Computer Networks is a numerical-heavy GATE CS subject. These lessons explain each layer with figures, work through the standard calculations (delays, efficiency, window sizes, subnets, fragmentation, congestion windows) step by step, and use small Python programs to compute CRCs, checksums, subnets and routes, so every number shown is checked.

Start the first lesson

1Basics

  1. Introduction and LayeringWhat a network is, the OSI and TCP/IP models, encapsulation, which device works at which layer, and the four kinds of delay.
  2. Circuit and Packet SwitchingCircuit switching, datagram packet switching and virtual circuits, store-and-forward delay over several hops, and why splitting a message into packets helps.

2Data Link Layer

  1. Framing and Error DetectionByte and bit stuffing, parity, the Internet checksum, CRC by polynomial division, Hamming distance and the number of check bits in a Hamming code.
  2. Flow Control and Sliding WindowsStop-and-wait, Go-Back-N and Selective Repeat; link utilisation, the window needed to fill a link, sequence number bits, and counting retransmissions.
  3. Medium Access Control and EthernetALOHA and slotted ALOHA throughput, CSMA and CSMA/CD, minimum frame size, Ethernet frames and binary exponential backoff, and how learning switches and bridges forward frames.

3Network Layer

  1. IPv4 Addressing, Subnets and CIDRIPv4 addresses, classful addressing, subnet masks, CIDR notation, subnetting and VLSM, route aggregation and longest prefix matching.
  2. The IP Protocol, Fragmentation, ARP, DHCP, ICMP and NATThe IPv4 header field by field, fragmentation and offsets, TTL, and the protocols that support IP (ARP, DHCP, ICMP, NAT), with a look at IPv6.
  3. Routing AlgorithmsForwarding versus routing, flooding, distance vector routing with Bellman-Ford and the count-to-infinity problem, link state routing with Dijkstra, and RIP, OSPF and BGP.

4Transport Layer

  1. Transport Layer, UDP, TCP and SocketsPorts and multiplexing, UDP, the TCP header, sequence and acknowledgement numbers, the three-way handshake and connection release, TCP flow control, sequence number wraparound, and socket programming.
  2. TCP Congestion ControlWhy congestion control is needed, the congestion window, slow start, congestion avoidance (AIMD), timeouts and triple duplicate ACKs in Tahoe and Reno, retransmission timer estimation, and throughput.

5Application Layer

  1. Application Layer ProtocolsDNS names, records and resolution, HTTP requests and responses with persistent and non-persistent connections and RTT counting, email with SMTP, POP3 and IMAP, and FTP.

6GATE Practice

  1. CN GATE Practice SetMixed GATE-style questions on delays, error detection, sliding windows, MAC, subnetting, fragmentation, routing, TCP and application protocols.