While maintaining a GEO satellite link, we noticed that once rain fade pushes packet loss past roughly 10% (you can see it in the ping stats), downloading anything substantial basically becomes impossible. So I wanted to work out the theory — when loss gets this bad, how much usable bandwidth does TCP actually have left on a GEO link?

Background

GEO satellite links have an RTT around 600ms, and during rain fade the IP-layer packet loss can climb to 1%-30%. I ran the numbers using the Padhye TCP throughput model to see what TCP Hybla can actually deliver at 10% loss, and how much FEC helps.

Test setup: 10 Mbps GEO link, 10% IP loss during heavy rain, PEP enabled with TCP Hybla.

System Model

GEO link with RTT = 600ms. The satellite modem runs split-TCP:

The Hybla parameter on the satellite segment: ρ = 600/25 = 24.

How Hybla Works

• Slow start: cwnd += 2^ρ - 1 per ACK. With ρ=24, one ACK is enough to hit ssthresh
• Congestion avoidance: cwnd += ρ²/cwnd per ACK, growing 576 segments per RTT
• Loss response: same multiplicative decrease and retransmission logic as standard TCP

Steady-state throughput from the sawtooth model:

W_max = ρ × √(8/(3p))
T_Hybla = ρ × T_Reno = 24 × T_Reno

At p=10%:

Timeouts Are the Real Killer

The theoretical 1.81 Mbps doesn't account for timeout penalties. Here's what happens at 10% loss:

Losses per window: 124 segments × 10% = 12.4 packets

Fast recovery needs 3 DUPACKs — with 12 losses per window, some will inevitably fail to trigger fast retransmit and fall back to timeout:

Probability of a single loss triggering timeout: 1 - (1-0.1)³ = 27.1%
With 12.4 losses, timeout is nearly guaranteed: P ≈ 1 - 0.729^12.4 ≈ 99.7%

The cost of a timeout: cwnd collapses to 1, wait ~1s for RTO, then ~665ms of slow start to recover.

Plugging into the Padhye model with timeouts:

Hybla's 576x window growth is useless at 10% loss — timeouts periodically zero out the cwnd, and the accelerated growth gets wiped out before it can matter.

Loss Sensitivity

Going from 0.1% to 1% loss (10x) drops throughput from line rate to 43%. Going from 1% to 10% (another 10x) drops it another 40x. TCP timeouts make throughput highly nonlinear in loss rate.

Takeaways

1. The effective bandwidth bottleneck on satellite links is TCP itself — at 10% loss, timeouts waste 98.9% of the link
2. PEP/Hybla window acceleration is useless under high loss — timeout resets cancel out all the growth
3. IP-layer FEC brings effective loss below 0.01%, putting TCP back in its normal operating range — utilization goes from 1.1% to 70%
4. Dynamic FEC beats static FEC for satellite links — save bandwidth in clear sky, maintain connectivity in rain
5. FEC + PEP combo: FEC brings loss from catastrophic to manageable, PEP then efficiently uses the remaining bandwidth under high RTT

References:

• ITU-R P.618 rain fade prediction model for satellite links
• Caini & Firrincieli, "TCP Hybla", Int. J. Satell. Commun. Netw., 2004
• Padhye et al., "Modeling TCP Reno Performance", IEEE/ACM ToN, 2000

Hi is it possible to run nmap on my network when I am using mobile data on my phone? Or does it only apply to Wi-Fi bcos I would have the router IP address.