[Home](./index.md) > [sip.js](./sip.js.md) > [UserAgentClient](./sip.js.useragentclient.md) > [onTransportError](./sip.js.useragentclient.ontransporterror.md) ## UserAgentClient.onTransportError() method 8.1.3.1 Transaction Layer Errors In some cases, the response returned by the transaction layer will not be a SIP message, but rather a transaction layer error. When a timeout error is received from the transaction layer, it MUST be treated as if a 408 (Request Timeout) status code has been received. 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 Signature: ```typescript protected onTransportError(error: TransportError): void; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | error | TransportError | Transport error | Returns: `void`