# A Research Study on Protocol Stack of Space-Based Optical Backbone Network
## 3 Design of the Protocol Stack
### Design of Control Plane
#### 3.1.3 Signal Protocol  
The signal protocol is used to build the route path calculated by the routing protocol. It can establish the LSP (label switching path), which transmits services according to labels. We used the CR-LDP (constraint-based routing label distribution protocol) as the signal protocol to build LPs for services. The label space is based on the platform, where a label is distributed for a destination network segment but not for an interface. In addition, we used a conservative mode to hold labels, which only reserves and maintains the labels used to forward data. Furthermore, according to the demands of protocol stack, we used DoD (distribution on demand) and ordered modes for label distribution and control, which are defined as follows:  
1. DoD Mode: used for label distribution, which sends a label mapping message to distribute the label to the upstream node only after having received a label request message from it. 2. Ordered mode: used for label control, which controls the processing of label distribution. It sends a label mapping message to the upstream node, only after having received a label mapping message from the downstream node. As shown in Figure 6, an LSP consists of a source node, a destination node and several LSRs (label switching routers), which can forward the service data according to the labels distributed by the signal protocol. According to the design of signal protocol, the process of building an LSP is detailed as follows:  
1. After the source node has received a requirement to build an LSP, it firstly performs the resource reservation operation. Then, it creates a label request message and sends this to the next downstream node. 2. After the downstream node has received the label request message, it firstly performs the resource reservation operation. Then, the node determines whether it is the destination node. 1. If the result is NO, the node forwards the label request message to the next downstream node. 2. If the result is YES, the node continues to perform the operations of resource utilization and label distribution. In the end, the node creates a label mapping message to the upstream node to notify the label distributed by it. 3. After the upstream node has received a label mapping message, it firstly performs the resource utilization operation. Then, the node determines whether it is the source node. 1. If the result is NO, the node continues to perform the label distribution operation. Then, it creates a label mapping message to the upstream node to notify the label distributed by it. 2. If the result is YES, the node replies with the information of the LSP. By this stage, the LSP has already been built. The functions of messages and operations mentioned above are defined as follows:  
1. Label Request Message: used to require the downstream node to distribute labels to the upstream node. 2. Label Mapping Message: used to send labels distributed by the downstream node to the upstream node. 3. Resource Reservation Operation: each link has a special value called RESERVED BANDWIDTH, which is used to record the usage of the link bandwidth by the link management protocol and is not used in the calculation of route path. That is to say, the changes of RESERVED BANDWIDTH do not affect the calculation of route path. So, we used this operation in the first half of processing to build the LSP. This operation deducs the required bandwidth from the RESERVED BANDWIDTH. 4. Resource Utilization Operation: each link has a special value called the AVAILABLE BANDWIDTH, which is used to record the usage of the link bandwidth by the link management protocol and is also used in the calculation of route path. So, changes to the AVAILABLE BANDWIDTH actually affect the calculation of route path. So, we used this operation in the latter half of processing to build the LSP, which deducs the required bandwidth from the AVAILABLE BANDWIDTH. 5. Label Distribution Operation: this operation is used to distribute labels to the LSP.