# A Research Study on Protocol Stack of Space-Based Optical Backbone Network
## 3 Design of the Protocol Stack
### Design of Control Plane
#### 3.1.1 Link Management Protocol  
The link management protocol is the base of the protocol stack. It has two main functions: control channel management and local resource collection. The function of control channel management establishes and maintains control channels, which transmit management and control messages. After the start of a node, each port sends Config messages periodically to negotiate for building control channels, until the ConfigAck message or ConfigNack message is received. 1. Receive the ConfigAck message: this means that the corresponding node has agreed on the content of negotiations sent by the Config message, including negotiated parameters such as Channel, HelloInterval, HelloDedInterval and so on. In this case, the control channel has already been built. 2. Receive the ConfigNack message: this means that the corresponding node has disagreed on the content of the negotiations sent by the Config message and replied the ConfigNack message, including the recommended negotiated parameters, which should be prioritized by this node. If this node disagrees on these recommended negotiated parameters, it could resend new negotiated parameters. After the control channels have been established, the link management protocol also needs to maintain them by periodically sending Hello messages in each control channel. If a port has not received any Hello message from a control channel within the fixed time (HelloDedInterval), the port will determine that the control channel is invalid. The functions of the messages and parameters mentioned above are defined as follows:  
1. Config message: used to negotiate for the establishment of control channels, including parameters about the control channels. 2. ConfigAck message: used to reply to the Config message while the node has agreed on the negotiated parameters. 3. ConfigNack message: the message--including the recommended parameters--is used to reply to the Config message when the node disagrees with the negotiated parameters. 4. Hello message: this message is sent periodically to maintain the control channels built by the link management protocol. 5. Channel: used to confirm the parameters of a control channel, such as specific wavelength, specific field and so on. Here, we used specific field to realize control channels. 6. HelloInterval: used as the interval at which the Hello messages are sent. 7. HelloDeadInterval: used as the maximum time to decide whether a control channel is invalid. If the port has not received any Hello message within the time, the control channel is invalid. Based on the control channels established above, several messages, such as the LinkSummary message, LinkSummaryAck message and LinkSummaryNack message, could be transmitted to implement the function of local resource collection. This could collect and maintain local resources, which could be used by a routing protocol to calculate route paths. The port would send LinkSummary messages periodically, including link-related parameters such as bandwidth, port rate and so on, until it has received the LinkSummaryAck message or LinkSummaryNack message. Receive the LinkSummaryAck message: means that the corresponding node has agreed on the link-related parameters sent by the LinkSummary message. In this case, the link parameters have been confirmed and will be collected by the link management protocol. Receive the LinkSummaryNack message: means that the corresponding node has disagreed on the link-related parameters sent by the LinkSummary message and replied the LinkSummaryNack message, including stating that the parameters could not be accepted. This node could resend new parameters again until they both come to an agreement. The functions of messages mentioned above are defined as follows:  
1. LinkSummary message: used to confirm the link-related parameters. 2. LinkSummaryAck message: used to reply to the LinkSummary message when the node agrees on the link-related parameters. 3.