400
SIP
400 Bad Request
The request could not be understood due to malformed syntax. The client should not repeat the request without modifications.
On this page
When You See This Code
When the SIP message has syntax errors, missing required headers, or invalid header values.
How to Fix
Review the SIP message for syntax errors. Check required headers (To, From, Call-ID, CSeq, Via, Max-Forwards) and ensure proper formatting.
Specification
Cross-Protocol Equivalents
Compare With
Guides
401 Unauthorized vs 403 Forbidden: When to Use Each
Security & Authentication
SIP Response Codes: Understanding VoIP Call Flow
Real-Time Protocols
SIP Call Flow: Understanding Session Setup
Real-Time Protocols
WebRTC Signaling with SIP and WebSocket
Real-Time Protocols
RFC 3261: SIP Protocol Deep Dive
Protocol Deep Dives
TLS 1.3: Faster, Simpler, More Secure
Protocol Deep Dives
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TLS Versions
Frequently Asked Questions
What is SIP 400 Bad Request?
The request could not be understood due to malformed syntax. The client should not repeat the request without modifications.
When do you see SIP 400 Bad Request?
When the SIP message has syntax errors, missing required headers, or invalid header values.
How to fix SIP 400 Bad Request?
Review the SIP message for syntax errors. Check required headers (To, From, Call-ID, CSeq, Via, Max-Forwards) and ensure proper formatting.