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.