IP Datagram Structure Overview

Overview

This lecture covers the structure and fields of an IP datagram, explaining their functions and how network data is encapsulated in layers.

IP Datagram Structure

• An IP datagram consists of two main sections: the header and the payload.
• The header contains more data than an Ethernet frame header and has several specific fields.

Key Header Fields

• The version field (4 bits) specifies the IP version (IPv4 or IPv6).
• The header length field (4 bits) indicates the total header size, usually 20 bytes for IPv4.
• The service type field (8 bits) can specify Quality of Service (QoS) for prioritized routing.
• The total length field (16 bits) shows the full size of the IP datagram; max size is 65,535 bytes.
• The identification field (16 bits) groups related datagrams for reassembly if fragmentation occurs.
• The flag field indicates whether fragmentation is allowed or has occurred.
• The fragmentation offset field helps reassemble fragmented datagrams in the correct order.
• The time to live (TTL) field (8 bits) decreases by 1 at each router to prevent infinite looping.
• The protocol field (8 bits) specifies the transport layer protocol (e.g., TCP or UDP).
• The header checksum field provides error checking for the header and is recalculated at each router.
• The source and destination IP address fields (each 32 bits) contain sender and receiver IP addresses.
• The IP options field is optional and can add special characteristics, mostly for testing.
• The padding field ensures the header reaches the required length with extra zeros if needed.

Encapsulation & Networking Layers

• An IP datagram is encapsulated as the payload within an Ethernet frame.
• The IP datagram payload contains the entire TCP or UDP packet.
• Networking layers depend on each other, with each layer providing the foundation for the next.

Key Terms & Definitions

• IP Datagram — a packet at the network layer under the IP protocol.
• Header — structured fields containing essential routing and delivery information.
• Payload — the data section, often a TCP or UDP packet.
• Fragmentation — splitting a datagram into smaller parts to traverse networks with smaller maximum sizes.
• Encapsulation — enclosing data from one layer into a packet of the lower layer.
• TTL (Time to Live) — limits a datagram's lifetime to prevent endless looping in the network.

Action Items / Next Steps

• Review the structure and functions of each IP datagram header field.
• Prepare for questions about encapsulation and the relationship between different networking layers.