Quartz 5

Networks 3

6 min read

Checksums

Definition

A checksum is an error detection method used by most TCP/IP protocols.

Purposes

  • Protects against corruption during packet transmission.

    • Calculated at sender and sent with packet.
    • Recipient host performs the same calculation and accepts the packet if it is correct.

Sender Process

  1. Packet is divided into k sections of n bits each.
  2. All data undergoes one’s compliment arithmetic, where the the data is summed and any overflow is wrapped.
  3. Added together using one’s complement arithmetic.
  4. Result is complemented(bits flipped) to form the checksum.

Recipient Process

  1. Received packet (k sections of n bits) undergoes the same process as the sender.
  2. result of arithmetic is added to checksum and complemented
  3. If the final result = 0, the packet is accepted.

One’s Complement Arithmetic

  • Binary addition, but the carry out of the most significant bit is wrapped around and added back into the least significant bit.

IP Header and Checksum

  • IP checksum covers only the header, not the data.

  • Higher-level protocols encapsulated in IP packets have their own checksum fields that cover the entire packet.

  • Routers only need to recalculate the checksum for the header, not the data.

    • If the data were included, routers would have to recalculate the checksum for the whole packet → less efficient.

Transport Layer

Definition

Ensures process-to-process delivery of messages between different applications on the host.

Purposes

  • Service point addressing – Port numbers direct data to the correct application.

  • Segmentation and reassembly –

    • Messages split into smaller segments.
    • Segments assigned sequence numbers.
    • Destination transport layer reassembles in correct order.
    • Missing segments are identified and retransmitted.

  • Connection control

    • Connection-based (TCP) – Establishes and terminates session before and after transfer; ensures ordered delivery.
    • Connectionless (UDP) – No setup; packets sent independently.

  • Flow control – Regulates transmission rate; prevents overload.

  • Error control – Detects and corrects errors using acknowledgements and retransmission.

User Datagram Protocol (UDP)

Faster method for transmitting small amounts of data.

  • Connectionless – No handshake.
  • Unreliable – No guarantee of delivery order or success.
  • Minimal overhead – 8-byte header.

Checksum

  • Covers: pseudo header + UDP header + data.
  • Pseudo header comes from the IP packet header.

Applications of UDP

  • Real-time applications: video calls, online gaming.

  • Lightweight services:

    • DNS (port 53).
    • TFTP – Trivial File Transfer Protocol (port 69).

  • Broadcast / multicast services:

    • mDNS (port 5353).

Transmission Control Protocol (TCP)

Definition

A connection-oriented protocol that provides reliable, ordered, and error-checked delivery of data between applications over a network.

  • Slower transfer but reliable for large amounts of data.
  • Connection-oriented – Handshake establishes reliable connection.
  • Reliable delivery – Integrity ensured via acknowledgements + timeouts.
  • Error checking – Checksum.
  • Flow control – Prevents overwhelming the receiver.
  • Congestion control – Manages load, prevents congestion.
  • Ordered delivery – Segments are reassembled using sequence numbers.
  • Fixed header size – 8 bytes.

TCP Mechanisms

Connection Establishment – Three-Way Handshake

  • Sender and receiver agree on connection state.
  • Three messages exchanged: SYN → SYN-ACK → ACK.
  • Uses an initial sequence number (ISN) chosen randomly.

Connection Termination – Four-Way Handshake

  • Supports half-close (one side stops sending but continues receiving).
  • Either server or client can initiate termination.
  • Ensures all data is delivered and acknowledged.

Sequence Numbers

  • Every byte has a sequence number.

  • Sequence number = the first byte of user data in a segment.

  • Used for:

    • Reordering data.
    • Detecting missing data.

Acknowledgements (ACK)

  • Receiver sends ACK with the next expected byte number.
  • ACK = sequence number of last byte received + 1.
  • Both sender and receiver maintain ACKs for reliable delivery.

Retransmissions

  • Each sent segment has a retransmission timer.
  • If timer expires before ACK → segment is resent.
  • Fast recovery – Segment is retransmitted immediately upon receiving 3 duplicate ACKs.

Checksum

  • 16-bit checksum covers header + data.
  • Detects corruption → corrupted segment discarded and retransmitted.

Flow Control – Sliding Window

  • Sender specifies window size = how much data can be sent before needing an ACK.

  • Sliding window mechanism:

    • Allows transmission of multiple packets before waiting for ACKs.

    • Operates at octet level.

    • Window size changes dynamically:

      • ACK contains receiver’s buffer limit.
      • If receiver buffer = 0 → sender stops transmitting.

  • Prevents buffer overflow at receiver.

Ordered Delivery

  • Out-of-order segments are cached until missing ones arrive.
  • Data is delivered to application only after reordering.

Port Numbers

  • Well-Known Ports (0–1023): Used by standard services (HTTP, FTP, DNS, etc.).
  • Registered Ports (1024–49151): Used by user applications.
  • Dynamic/Ephemeral Ports (49152–65535): Assigned temporarily for client connections.

TCP Mechanisms

Connection Establishment – Three-Way Handshake

  • Sender and receiver agree on connection state.
  • Three messages exchanged: SYN → SYN-ACK → ACK.
  • Uses an initial sequence number (ISN) chosen randomly.

Connection Termination – Four-Way Handshake

  • Supports half-close (one side stops sending but continues receiving).
  • Either server or client can initiate termination.
  • Ensures all data is delivered and acknowledged.

Sequence Numbers

  • Every byte has a sequence number.

  • Sequence number = the first byte of user data in a segment.

  • Used for:

    • Reordering data.
    • Detecting missing data.

Acknowledgements (ACK)

  • Receiver sends ACK with the next expected byte number.
  • ACK = sequence number of last byte received + 1.
  • Both sender and receiver maintain ACKs for reliable delivery.

Retransmissions

  • Each sent segment has a retransmission timer.
  • If timer expires before ACK → segment is resent.
  • Fast recovery – Segment is retransmitted immediately upon receiving 3 duplicate ACKs.

Checksum

  • 16-bit checksum covers header + data.
  • Detects corruption → corrupted segment discarded and retransmitted.

Flow Control – Sliding Window

  • Sender specifies window size = how much data can be sent before needing an ACK.

  • Sliding window mechanism:

    • Allows transmission of multiple packets before waiting for ACKs.

    • Operates at octet level.

    • Window size changes dynamically:

      • ACK contains receiver’s buffer limit.
      • If receiver buffer = 0 → sender stops transmitting.

  • Prevents buffer overflow at receiver.

Ordered Delivery

  • Out-of-order segments are cached until missing ones arrive.
  • Data is delivered to application only after reordering.

Port Numbers

  • Well-Known Ports (0–1023): Used by standard services (HTTP, FTP, DNS, etc.).
  • Registered Ports (1024–49151): Used by user applications.
  • Dynamic/Ephemeral Ports (49152–65535): Assigned temporarily for client connections.

TCP Mechanisms for Reliable Data Transmission

  • Sequence Numbers

    • Every byte is assigned a sequence number.
    • Allows reordering and detection of missing data.

  • Acknowledgements (ACKs)

    • Receiver sends an ACK with the next expected byte number.
    • ACK = last received byte + 1.
    • Confirms correct, in-order delivery.

  • Retransmission

    • Sender sets a retransmission timer for each segment.
    • If ACK not received, the segment is resent.
    • Fast recovery: sender retransmits after 3 duplicate ACKs.

  • Checksum

    • 16-bit checksum covers header + data.
    • Corrupted segments discarded and considered lost.

  • Flow Control

    • Receiver advertises a window size to limit sender.

    • Prevents buffer overflow.

    • Uses sliding window protocol:

      • Multiple packets sent before waiting for ACKs.
      • Operates at octet level.
      • Window size dynamic (ACK updates).
      • 0 → sender stops transmission.

  • Ordered Delivery

    • Out-of-order segments cached.
    • Delivered in sequence to the application.

Port Numbers



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