1. Can message traffic be sent without both side exchanging Good-Day messages first?
Yes, the SMS transport message header of every message can be used as Good- Day.
2. When the SCP sends the GoodDay, must the DestNodeName be blank (zero length)?
3. If the SCP already knows the DestNode Name, can it fill in the DestNode Name?
If they fill in DestNode, no GoodDay will be returned. If they leave DestNode blank, a Good Day with filled DestNode must be returned.
4. When the SMS receives the GoodDay, must it respond with a good-day?
Only if the DestNode is blank.
5. SMS will listen for connections after the connection is established, and if SMS does not see the Good-day from SCP, will SMS send the good-day? (and I assume the SCP MUST response with a GoodDay)
SMS will send GoodDay every time the connection is established although it is not necessary.
6. What is the difference between goodbye and goodnight?
GoodBye is used for graceful shutdown by flushing all messages in the buffer.
GoodNight is to shutdown immediately, some buffered messages may be lost.