TCP Segment TCP

A TCP segment — header plus application data.

Reading a capture

Seven layers of headers have now been added to whatever the application actually wanted to send. Counting them is the fastest way to understand why small packets are expensive on a wireless network.

Subfields

• Source Port — The sending endpoint's port.
• Destination Port — The receiving endpoint's port.
• Sequence Number — Position of the first data octet in this segment within the stream.
• Acknowledgment Number — The next sequence number the sender of this segment expects.
• Data Offset — Header length in 32-bit words. Five means no options.
• Flags — The eight control bits.
• Window — How many more octets the sender of this segment is willing to accept.
• Checksum — Covers the header, the data, and a pseudo-header built from the IP addresses above.
• Urgent Pointer — Offset to urgent data, when URG is set.

Where it sits

TCP Segment

• Open in the explorer — TCP over IPv4, in a protected QoS Data frame, aggregated
• 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

Accuracy

Clause

RFC 9293 §3.1

Verified

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