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TCP Window Size Lab

BANDWIDTH-DELAY PRODUCT (BDP) ANALYSIS
OPTIMAL TCP WINDOW (BDP)
610KB
625,000 BYTES IN FLIGHT
SENDER (TX)BANDWIDTH-DELAY PRODUCT: 610 KBRECEIVER (RX)
DYNAMICS: AT 100Mbps WITH 50ms LATENCY, THE PIPELINE CONTAINS 625,000 BYTES AT ANY INSTANT.

The Physics of "In-Flight" Data

The Bandwidth-Delay Product (BDP) is the volume of data that can be present in a network "pipe" at any given time. If the TCP Receive Window is smaller than the BDP, the sender will stop and wait for acknowledgments before sending more data, effectively throttling the connection.

BDP (bits)=Bandwidth (bps)×RTT (sec)\text{BDP (bits)} = \text{Bandwidth (bps)} \times \text{RTT (sec)}

For high-latency satellite links or trans-continental fiber, standard 64KB TCP windows are insufficient. Calculating the BDP is the first step in TCP Tuning and enabling Window Scaling options.

ENGINEER'S NOTE

"I once saw a 10Gbps link performing at 10Mbps. The RTT was 180ms, but the window size was capped at 64KB. By tuning the window to match the BDP (~220MB), we restored the full throughput. Latency kills bandwidth if the window is small."

Understanding Long Fat Networks (LFN)

The 'Pipe' Analogy

Think of a network connection as a pipe. The bandwidth is the diameter of the pipe, and the RTT is the length. To keep the pipe full, the amount of water (data) inside must equal the volume of the pipe. If your TCP Receive Window is smaller than this volume, you are essentially leaving the pipe empty for part of the transmission cycle.

TCP Window Scaling (RFC 1323)

Original TCP headers limited the window to 64KB. For modern high-speed relative to latency, we use Window Scaling to increase this limit up to 1GB.

  • Calculate Bandwidth-Delay Product (BDP) for your worst-case RTT.
  • Enable `net.ipv4.tcp_window_scaling` on Linux servers.
  • Ensure buffer sizes on intermediate switches can handle the burst.

Technical Standards & References

REF [RFC-1323]
V. Jacobson, R. Braden, D. Borman (1992)
TCP Extensions for High Performance
“The definitive standard for TCP window scaling and BDP optimization.”
VIEW OFFICIAL SOURCE
REF [LFN-01]
ESnet (Energy Sciences Network) (2021)
Tuning TCP for Long-Fat Networks
“Authoritative resource for high-bandwidth, high-latency network throughput tuning.”
VIEW OFFICIAL SOURCE
REF [BDP-IEEE]
IEEE Communications Magazine (2018)
Performance Analysis of TCP over High-Speed Links
“Analysis of the relationship between buffer sizes and the Bandwidth-Delay Product.”
Mathematical models derived from standard engineering protocols. Not for human safety critical systems without redundant validation.

Related Engineering Resources

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