## 2 Background
### Hybrid Wireless Mesh Protocol
#### 2.2.2 Operation Mode [23]  
IEEE 802.11s defines HWMP as a basic routing protocol for a WMN. HWMP is a hybrid routing protocol that combines a reactive mode and a proactive mode. The protocol's reactive mode operation is based on AODV, while the proactive mode uses tree-based routing. HWMP is located on layer 2; therefore, it uses MAC addresses instead of IP addresses to route message communications. In addition, note that the term 'path selection' is used instead of 'routing' in layer 2 routing. The main purpose of the on-demand routing protocol is to support the mobile mesh points, while the proactive routing protocol supports the fixed nodes. The airtime metric is a default routing metric that is used to measure the link quality. In HWMP, on-demand routing and proactive routing can simultaneously operate. HWMP sets up an MP as a root MP to build the proactive tree. Three different methods for proactive tree-building in HWMP are shown in Figure 6. The first method only uses the PREQ mechanism. The second method uses the PREQ and PREP mechanisms together. The third method uses the RANN, PREQ, and PREP mechanisms. If a MP is configured as a root MP, at least one of the proactive PREQ and RANN mechanisms should be configured. In the proactive PREQ mechanism, the proactive PREP flag can be set or not. When the proactive PREP flag is not set, then only the proactive PREQ mechanism is used to build the proactive tree. Figure 5: MF and NMF of PREQ, PREP, RANN, and PERR. If a root MP is configured to use the proactive PREQ mechanism, then it will periodically broadcast a proactive PREQ with an increasing sequence number. Any intermediate MPs that receive a proactive PREQ will process it in similar way as the PREQ mechanism performs on-demand path discovery. The proactive PREP flag in the PREQ controls whether the PREP is sent in response to a proactive PREQ. If the flag is not set, then no PREP is sent in response to the reception of a proactive PREQ. This situation is called the non-registration mode. In this mode, a path tree from all of the MPs to the announced root MP is established, but the MPs are not registered proactively at the root MP. If a source MP attempts to establish a bidirectional communication path with the root MP, the source MP can send a gratuitous PREP before the first data frame to register its address with the root MP. The non-registration mode creates a path that does not strain the network and thus maintains proactive paths to the root MP. If the proactive PREP flag is set, then the MP must send the PREP in response to the reception of a proactive PREQ. This situation is called the registration mode. In this case, MPs register with the root MP by sending a PREP in response to the proactive PREQ. If a root MP is configured to use the proactive RANN mechanism, then it will periodically broadcast a RANN with an increasing sequence number. The RANN mechanism propagates only the path metrics to the root MP and all of the MPs in the mesh network. The RANN mechanism does not create or update any paths in the routing table. If an MP wants to create or update a path to the root MP, then it will send a unicast PREQ to the root MP, and the root MP will respond with a PREP. PREQ/PREP processing is performed in a similar manner as PREQ/PREP processing during on-demand path discovery. This mechanism establishes a forwarding tree for each MP toward the root MP. Multiple root MPs can be configured in a mesh network that is running HWMP, which means that multiple proactive trees can be built simultaneously by the different root MPs. Figure 6: Configuration cases for HWMP [23]. The hybrid routing event occurs when a root MP is configured as a registration mode. When a source MP wants to send data to a destination MP but has no path to the destination in its routing table, the source can send the data frames to the root MP. Because the mesh network is in registration mode, the root MP knows that the destination is inside of the mesh network.