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Chapter 7: Internet Protocol Version 4 (IPv4)

Key Concepts from Chapter 7: Internet Protocol Version 4 (IPv4)

Objectives of the Chapter

  • Explain the role of the IP protocol within the TCP/IP suite.
  • Understand the IPv4 datagram structure.
  • Discuss datagram fragmentation and reassembly.
  • Learn about options within an IPv4 datagram and their applications.
  • Calculate checksums for IPv4 datagram headers and their verification at the receiver.
  • Explore IP over ATM vs. IP over LANs/point-to-point WANs.
  • Understand a simplified IP package and the pseudocode for relevant modules.

Introduction to IP

  • The Internet Protocol (IP) operates at the network layer of the TCP/IP protocol suite, essential for routing and delivering packets across networks.

Format of an IPv4 Datagram

  • A datagram is a variable-length packet consisting of a header (20-60 bytes) and data (20-65,535 bytes).
  • Header Structure:
  • Version: 4 bits (indicating IPv4)
  • Header Length (HLEN): 4 bits
  • Total Length: 16 bits (header + data)
  • Identification: 16 bits
  • Flags: 3 bits (indicating fragmentation)
  • Time to Live (TTL): 8 bits
  • Protocol: 8 bits (denotes the transport layer protocol)
  • Source IP Address: 32 bits
  • Destination IP Address: 32 bits
  • Options: Up to 40 bytes, can include various control information.

Fragmentation of Datagrams

  • Fragmentation is necessary when datagrams exceed the Maximum Transfer Unit (MTU) of the network.
  • MTU and Fragmentation Fields:
  • Fragmentation only applies to data; the header remains intact.
  • Flags:
    • D (Do not fragment): blocks fragmentation.
    • M (More fragments): indicates if more fragments follow.
  • Upon fragmentation, each fragment contains a partial datagram plus necessary header info to ensure it can be reassembled correctly.

Options in IPv4 Datagram

  • Options are not mandatory but may include elements such as timestamp, record route, strict source route, and loose source route for testing/debugging purposes.
  • They can comprise a maximum of 40 bytes, divided into single-byte or multiple-byte formats.

Checksum Calculation

  • The checksum is used for error detection in the header of the IP datagram (not the data).
  • Checksum Calculation:
  • At sender: All fields are added in 16-bit segments, then complemented and inserted.
  • At receiver: The same calculation is performed, and if the result is all zeros, the packet is accepted.

IP Over ATM

  • Explains encapsulation in ATM networks where IP packets are converted into ATM cells.
  • Also introduces the ATMARP protocol for binding IP addresses to physical addresses in ATM networks.

Security Implications

  • Originally, IPv4 lacked security features; however, it is now crucial to consider security due to modern threats.
  • Discusses additional protocols like IPSec for providing security features.

IP Package Structure

  • Describes a simplified model of how an IP datagram is processed and the modules involved, such as:
  • Header-Adding Module: To encapsulate data and insert checksums.
  • Processing Module: To manage incoming datagrams (routing, fragmentation, etc.).
  • Reassembly Module: To reconstruct fragmented datagrams.