[Home](./index.md) > [sip.js](./sip.js.md) > [TransactionUser](./sip.js.transactionuser.md) > [onTransportError](./sip.js.transactionuser.ontransporterror.md) ## TransactionUser.onTransportError property Callback for notification of a transport error. If a fatal transport error is reported by the transport layer (generally, due to fatal ICMP errors in UDP or connection failures in TCP), the condition MUST be treated as a 503 (Service Unavailable) status code. https://tools.ietf.org/html/rfc3261\#section-8.1.3.1 https://tools.ietf.org/html/rfc3261\#section-17.1.4 https://tools.ietf.org/html/rfc3261\#section-17.2.4 https://tools.ietf.org/html/rfc6026 Signature: ```typescript onTransportError?: (error: TransportError) => void; ```