actix/actix-web · error · ProtocolError

unknown continuation fragment: {}

Error message

unknown continuation fragment: {}

What it means

ProtocolError::ContinuationFragment(OpCode) (ws/mod.rs:63) is raised in codec.rs:257-260 when a non-final (FIN=0) frame arrives whose opcode is not Continue, Binary, or Text. Per RFC 6455 §5.4 only data frames may be fragmented; a fragmented control frame (Close/Ping/Pong) or a reserved opcode in a fragment is illegal, so actix reports the offending opcode.

Source

Thrown at actix-http/src/ws/mod.rs:62

    /// Bad opcode.
    #[display("bad opcode")]
    BadOpCode,

    /// A payload reached size limit.
    #[display("payload reached size limit")]
    Overflow,

    /// Continuation has not started.
    #[display("continuation has not started")]
    ContinuationNotStarted,

    /// Received new continuation but it is already started.
    #[display("received new continuation but it has already started")]
    ContinuationStarted,

    /// Unknown continuation fragment.
    #[display("unknown continuation fragment: {}", _0)]
    ContinuationFragment(#[error(not(source))] OpCode),

    /// I/O error.
    #[display("I/O error: {}", _0)]
    Io(io::Error),
}

/// WebSocket handshake errors
#[derive(Debug, Clone, Copy, PartialEq, Eq, Display, Error)]
pub enum HandshakeError {
    /// Only get method is allowed.
    #[display("method not allowed")]
    GetMethodRequired,

    /// Upgrade header if not set to WebSocket.
    #[display("WebSocket upgrade is expected")]
    NoWebsocketUpgrade,

View on GitHub (pinned to 937960ca67)

Solutions

1. Close the connection; the peer violates the fragmentation rules of RFC 6455.
2. Fix the peer implementation to always set FIN=1 on Ping/Pong/Close frames.
3. Verify no intermediary is re-segmenting WebSocket frames.

// Handle ProtocolError from the WS dispatcher and close cleanly.
if let Err(ProtocolError::ContinuationFragment(op)) = dispatch_result {
    log::error!("illegal fragmented opcode {op}; tearing down connection");
    // close the actor; recovery is impossible mid-fragment
}

When it happens

Trigger: A WebSocket peer sends an unfinished (FIN bit clear) Ping, Pong, or Close frame, or interleaves a control opcode into a fragmented message incorrectly. The codec's !finished branch (codec.rs:226-261) falls through to the error arm.

Common situations: Buggy client that fragments control frames, fuzz testing, or a proxy that splits frames incorrectly. Legitimate browsers never fragment control frames.

Related errors

• invalid opcode ({})
• invalid control frame length ({})
• Invalid chunk size line: Invalid Size
• Invalid chunk size linear white space
• Invalid chunk size LF

AI-assisted analysis of actix/actix-web@937960ca67 (2026-08-06). Data as JSON: /data/errors/ad26e9dee705e567.json. Report an issue: GitHub.