3.2.4.5.13 Echo Request PDU
An outbound proxy implementation MUST NOT accept the Echo Request PDU in any state other than Open_Start. If this condition is not met, the outbound proxy implementation MUST treat it as a protocol error as specified in section 3.2.4.5.14.
If this PDU is received in an Open_Start state, then the outbound proxy implementation MUST perform the following actions in the sequence given:
Send an Echo Response PDU to the client.
Transition to the Finished state.