TCP Port Exhaustion
TCP port exhaustion occurs in distributed training when many connections exhaust ephemeral port range.
TCP port exhaustion occurs in distributed training when many connections exhaust ephemeral port range.
What this failure is
TCP Port Exhaustion is a Communication failure seen during ML training runs. TCP port exhaustion occurs in distributed training when many connections exhaust ephemeral port range. Common tags: Tcp, Port, Exhaustion, Network.
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Why it happens (the mechanism)
Ephemeral port range exhausted. Too many TIME_WAIT sockets. NCCL doesn't reuse connections. Container network port range too small. Taken together, these mechanisms explain why the failure is reproducible, why it tends to surface on specific workloads or scales, and why generic mitigation attempts often fall short without addressing the underlying cause.
What you'll observe
- Distributed training fails to bind ports
- Connection refused errors at scale
- NCCL fails to establish sockets
Common symptoms and what they mean
| Symptom | Why it happens |
|---|---|
| OSError: [Errno 99] Cannot assign requested address | Ephemeral port range exhausted |
| NCCL WARN connect: Connection refused | Too many TIME_WAIT sockets |
| bind: Address already in use | NCCL doesn't reuse connections |
Which systems are affected
- Large-scale distributed training (64+ nodes)
- Many concurrent TCP connections
- Kubernetes services with high connection churn
How to confirm this is the problem
Use this checklist to test the hypothesis against a small reproduction. No single line proves the root cause, so preserve the preceding events and compare one variable at a time.
- ✓Reproduce the failure from a clean checkpoint/seed: the symptom must appear without warm-up state from a previous run.
- ✓Verified signal present: OSError: [Errno 99] Cannot assign requested address
- ✓Verified signal present: NCCL WARN connect: Connection refused
- ✓Verified signal present: bind: Address already in use
- ✓A targeted fix from the "How to fix it" section eliminates or substantially reduces the symptom within one validation pass.
The fix and the prevention pattern
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Diagnose this failure in VS Code
Select the traceback or open the failed terminal, then run Denpex locally to see the initiating rank, collateral failures, exact fix, and verification command without uploading the log.
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Root cause
- Ephemeral port range exhausted
- Too many TIME_WAIT sockets
- NCCL doesn't reuse connections
- Container network port range too small
The fix and how to prevent it
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