Quick reference
| Number | What | Why it matters | |
|---|---|---|---|
| 1500 | Ethernet MTU | The payload a standard frame carries; 1460 is TCP's MSS. | mtu |
| 1 RTT | TCP handshake | SYN, SYN-ACK, ACK before data; the third leg can carry it. | tcp |
| 1 RTT | TLS 1.3 handshake | One round trip after TCP, so the first byte waits two. | tls |
| 5 MB | BDP at 1 Gbps, 40 ms | Bandwidth x RTT / 8: the data in flight to fill the pipe. | bdp |
| 150 ms | Cross-region RTT | Same continent 10-40 ms; across an ocean 150-250 ms. | rtt |
| 1% | Packet loss | Enough to cut a 40 ms path to single-digit Mbps. | loss |
Explained
Size
1500 bytes is the Ethernet MTU, the payload a standard frame carries, and 1460 is what TCP gets after 20 bytes of IP and 20 of TCP header. Jumbo frames push the MTU to 9000 on a controlled LAN, and every hop must agree or the path falls back to 1500.
Round trips
A TCP handshake costs 1 RTT before data, and TLS 1.3 costs 1 more, so the first byte waits 2 RTT and the request waits a third.
The pipe
The BDP at 1 Gbps and 40 ms is 5 MB, the data in flight needed to fill the link. A 64 KB window there allows about 13 Mbps, which is why a long path with a small window looks like a slow link.
Distance and loss
Cross-region RTT is 150 ms across an ocean, against 10-40 ms on the same continent, so three round trips cost 450 ms before the server has done any work. A 1% packet loss can pull a 40 ms path into single-digit Mbps, because TCP reads loss as congestion and cuts the window.
The honest caveat: these are round numbers from textbook paths. Real RTT depends on routing, peering and queueing, and your 150 ms may be 90 on a good day.
The lesson: put a number on the path before you blame the application.
In one line
Distance costs round trips. Every handshake, window and retransmit is paid in RTT.
Measure one path
mtr -rw api.example.comMore notes: Networking & DNS
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