TCP window

How it works

Throughput cannot exceed window divided by round-trip time. A small window on a satellite or cross-ocean path leaves bandwidth idle. RFC 6349 treats this as a test item. File tools rarely expose it. The symptom is a fast LAN and a slow long-haul copy on a fat pipe.

A 10 GB copy across a 90 ms path holds 20 Mbps on a 1 Gbps circuit. Raising the window and enabling window scaling moves it to 400 Mbps. Buying more bandwidth would have done nothing.

How it differs

The TCP window is not the file size. It is in flight, not total.

A middlebox that clamps windows recreates the cap.

Test the path, not only the server.

On the ticket

• The practical close is a log line: time, actor, byte count, result.
• Without that line the transfer is a story.
• With it, the next person can see whether this door did what the ticket claimed.
• If the path is shared, say so in the partner profile so a later change does not silently pick a different limit, key, or region.
• On a real ticket, write down the door, the byte count, and the clock.
• For tcp window, that means naming the host or bucket, the expected size, and the time the other side must have a complete file.
• A progress bar is not that record.
• A 200 response that arrives before the complete call is not that record.
• If a retry is allowed, say how many and whether it resumes.
• If a person must approve the send, name the person.
• Partners who receive tcp window files should match on hash or size before they import.
• A same-length corrupt file passes a size check and fails a hash.
• Keep the published hash off the only channel an attacker can edit, or treat it as a corruption check rather than a substitution check.
• When the path changes, new key, new region, new cap, update the profile the same day so the next run does not use a stale limit.