How to fix Error 4080 on Windows Server 2025 / Windows 11

by

Troubleshooting Guide: SQL Server Error 4080 – Incorrect TDS Protocol Stream

Why This Error Happens

Error 4080 occurs when the SQL Server Native Client or ODBC driver sends a malformed TRACE TDS header, typically due to a mismatch between the client driver version and the SQL Server 2025 engine’s expected protocol version. This is commonly triggered by outdated connection drivers, corrupted network packets caused by MTU/packet fragmentation issues, or a misconfigured SQL Server Diagnostic Trace (Extended Events TDS tracing) that injects invalid header data into the stream. In some cases, third-party middleware or proxy software intercepting the TDS stream (such as load balancers) strips or alters header bytes, causing the engine to reject the connection.

Step-by-Step Resolution Guide

Step 1: Update and Verify Client Drivers

  • Confirm the client machine is using the latest ODBC Driver 19 (or newer) or OLE DB Driver for SQL Server, as older drivers are not fully compatible with SQL Server 2025’s TDS 8.0 protocol revisions.
  • Uninstall legacy “SQL Server Native Client (SNAC)” drivers if present, since they are deprecated and incompatible with newer TDS handshakes.
  • Re-run the connection string test using sqlcmd -S servername -U user -P password -C to validate basic connectivity before testing application-level connections.

Step 2: Disable Conflicting TDS Tracing or Extended Events

  • Open SQL Server Management Studio (SSMS) and navigate to Management > Extended Events > Sessions.
  • Check for any active sessions using the sqlserver.tds_trace or custom trace events; stop and delete sessions that may be malforming the header: DROP EVENT SESSION [TDS_Trace] ON SERVER;
  • Restart the SQL Server service via services.msc or run net stop MSSQLSERVER && net start MSSQLSERVER in an elevated Command Prompt.

Step 3: Inspect Network Path for Packet Fragmentation

  • Check if a firewall, proxy, or load balancer sits between the client and the SQL Server instance; bypass it temporarily to isolate the issue.
  • Verify the Network Interface Card (NIC) settings on Windows Server 2025 aren’t forcing Jumbo Frames mismatched with the switch MTU size (Control Panel > Network and Sharing Center > Adapter Settings > Advanced).
  • Run netsh int ipv4 show subinterfaces to check for MTU inconsistencies across the connection path.

Step 4: Reset the TDS Endpoint Configuration

  • Open SQL Server Configuration Manager, go to SQL Server Network Configuration > Protocols for MSSQLSERVER.
  • Disable and re-enable the TCP/IP protocol to force the endpoint to rebuild its listener configuration.
  • Confirm the TDS port (default 1433) isn’t being shared or intercepted by another service using netstat -ano | findstr :1433.

Alternative or Verification Steps

  • To verify the fix, establish a test connection via sqlcmd or SSMS and run a simple query (SELECT @@VERSION;) to confirm the TDS handshake completes without Error 4080 appearing in the SQL Server Error Log (Management > SQL Server Logs).
  • If the error persists, capture a Wireshark network trace filtered by tds protocol during the failed connection attempt to identify exactly which byte offset in the header is malformed, then compare it against Microsoft’s official TDS 8.0 specification document.