eason phrase is a well-formed response. A parser must accept this message. Only the space character after the reason code is required. If it were not present, this message could be rejected as invalid (a liberal receiver would accept it anyway). A simulation tool can be used to modify the reason phase C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security of the 200 response. The obtained message, inserted in a test case simulating a UAS, allows the simulation tool to respond to the INVITE sent by the IUT and test its behavior when a 200 response with an unusual or empty reason phase is received. Other stress messages can contain invalid scalar field headers. For example, in the INVITE client transaction case, the responses 200 or 300-699 to the INVITE could have a negative content-length. The IUT receiving these mes- sages should respond with an error. When a message like this appeared over UDP, the remainder of the datagram can simply be discarded. If a request like this arrives over TCP, the framing error is not recoverable, and the connection should be closed. The same behavior is appropriate for messages that arrive without a numeric value in the content-length header field. Scalar Fields with overlarge values could be used in 200 or 300-699 responses to the INVITE to stress the IUT. Scalar header field values outside their legal range are for example: - The CSeq sequence number >231. - The Max-Forwards value >255. - The Expires value >231 - The Contact expires parameter value >231 The IUT receiving this request should respond with a 400 bad request due to the CSeq error. If only the max-forwards field were in error, the element could choose to process the request as if the field were absent. If only the expiry values were in error, the element could treat them as if they contained the default values for expiration (3600 in this case). As a last torture test example, responses with missing required header fields can be considered. The IUT receiving a 200 or 300-699 containin