IPv4 Datagram IPv4

An IPv4 packet — header plus payload.

Reading a capture

Where the addressing stops being about radios and starts being about hosts. The MAC addresses three lanes above identify which stations are talking over the air; these addresses identify the endpoints the traffic is actually between, which may be nowhere near this network.

Subfields

• Version — Always 4 here.
• IHL — Header length in 32-bit words. Five means a twenty-octet header with no options.
• DSCP — Differentiated services code point — the traffic class.
• ECN — Explicit congestion notification.
• Total Length — Header plus payload, in octets.
• Identification — Groups the fragments of one original datagram.
• Flags — Reserved, Don't Fragment, More Fragments.
• Fragment Offset — Where this fragment sits in the original datagram, in 8-octet units.
• TTL — Hop limit. Decremented by each router; the packet dies at zero.
• Protocol — What the payload is.
• Header Checksum — Ones-complement checksum over the header only, not the payload.
• Source Address — Where the datagram came from.
• Destination Address — Where it is going.

Where it sits

IPv4 Datagram

• Open in the explorer — TCP over IPv4, in a protected QoS Data frame, aggregated
• Open in the explorer — ICMP echo over IPv4, in a QoS Data frame
• Open in the explorer — TCP over IPv4, in a protected QoS Data frame
• Open in the explorer — TCP over IPv4, in an open QoS Data frame
• Open in the explorer — UDP over IPv4, in a QoS Data frame

Accuracy

Clause

RFC 791 §3.1

Verified

Not verified against the standard. Treat this as a claim, not a fact.