IPv4 Header Format and Key Fragmentation/Integrity Fields (TTL, Header Checksum, Flags, Fragment Offset)

The IPv4 header is a fixed-length base header of 20 bytes (unless options are present). It contains control fields used for routing, fragmentation, and basic integrity checking. The diagram below shows the standard header layout and where the requested fields sit. 2

Key bit positions in the base header (first 20 bytes):

• TTL: 8-bit field at byte offset 8 (after Flags/Fragment Offset)
• Header Checksum: 16-bit field immediately following TTL
• Flags + Fragmentation Offset: packed into a 16-bit field split as 3-bit Flags and 13-bit Fragment Offset 2

The fields you asked about are involved in: preventing infinite routing loops (TTL), hop-by-hop header integrity (Header Checksum), controlling whether fragmentation is permitted and how fragments are reassembled (Flags), and specifying where a fragment’s payload belongs in the original datagram (Fragmentation Offset). 2

keyword

keyword IPv4 Header

IPv4 TTL field

The TTL (Time To Live) field is an 8-bit value. It is designed to limit the lifetime of an IPv4 datagram in the network so it does not circulate indefinitely. Each router decrements TTL by 1, and when TTL becomes 0, the router drops the packet (the TTL mechanism prevents routing loops from persisting forever).

What TTL changes affect:

• TTL changes at every hop, so the header content changes and the checksum must be updated.

Important learning terms:

• TTL
• hop
• router

IPv4 Header Checksum field

The Header Checksum field is a 16-bit checksum that covers the IPv4 header (not the payload). It is primarily used for error detection on the header while the packet moves hop-by-hop.

Since routers decrement TTL, the header changes, so the checksum must be recomputed by each router that forwards the packet.

Learning terms:

• Header Checksum
• one's complement
• integrity

IPv4 Flags field (DF/MF/Reserved)

The Flags are the top 3 bits of the 16-bit “Flags + Fragment Offset” field. They control fragmentation behavior. 2

Common interpretation:

• DF (Don’t Fragment): when set, fragmentation is not allowed.
• MF (More Fragments): when set, indicates that more fragments follow.
• Reserved: typically must be zero in modern usage (its exact meaning depends on the RFC framework and must not be used for general fragment control).

Why these matter:

• If a packet is too large for a path, the system decides whether to fragment or drop (depending on DF) and indicates whether the receiver should expect additional fragments (via MF).

Learning terms:

• Flags
• DF
• MF

IPv4 Fragmentation Offset field

The Fragmentation Offset is the lower 13 bits of the same 16-bit field as the Flags. It specifies the position of a fragment’s data within the original payload for reassembly. 2

Key details:

• The offset is measured in units of 8-byte blocks (i.e., each offset step corresponds to 8 bytes).
• The receiver uses these offsets (together with the Identification field) to place fragments in the correct order/positions during reassembly.

Learning terms:

• Fragmentation Offset
• fragment
• reassembly