## 2 Background
### Forwarding Information  
A HWMP sequence number is required to differentiate between the old and the new PREQ/PREP forwarding information. The sequence number is increased if a new PREQ frame is transmitted so that the other mesh STA can identify it as a new path request. The mesh STA will only process the PREQ frame if its sequence number is higher than the sequence number that it has saved in its routing table. If the sequence number is similar, the mesh STA will compare the path metric information and update the forwarding information if and only if the PREQ path metric is better. According to the drafted standards [1], the sequence number should not be increased too rapidly to avoid changing paths too often. ALM is the default metric used for IEEE802.11s. The ALM path metric is determined by cumulating the link metric value along a path. A path with a lower path metric indicates that it is a better path (_i.e._, it indicates a lower airtime cost or a lower transmission time). This metric measurement will influence the accuracy of the path quality measurement. If the measurement is accurate, efficient paths can be selected so that an optimum network performance can be achieved. \begin{table}
\begin{tabular}{c|c|c|c|c} \hline \multirow{2}{*}{**Field of** **Forwarding** **Information**} & \multicolumn{2}{c|}{**Received PREQ**} & \multicolumn{2}{c}{**Received PREP**} \\ \cline{2-5}  & **Forwarding** **Information for** **Transmitter** & **Forwarding** **Information for** **Originator Mesh STA** & **Forwarding** **Information for** **Transmitter** & **Forwarding** **Information for** **Target Mesh STA** \\ \hline HWMP  Sequence & Invalid if created, no changes if updated & Originator HWMP  Sequence Number & Invalid if created, no changes if updated & Target HWMP  Sequence Number \\ \hline Next Hop & Transmitter mesh STA & Transmitter mesh STA  address & Transmitter mesh STA  address & Transmitter mesh STA \\ \hline Path Metric & Accumulation of the initial value of the path & Accumulation of the value of the PREQ field & Accumulation of the initial value of the path & Accumulation of the value of the PREP  value of the metric with the metric  field metric with the  of the link to the  of the link to the  of the link to the & Accumulation of the link to the  of the link to the  of the link to & Accumulation of the link to \\ \hline Number of  Hops & 1 & Hopcount +1 & 1 & Hopcount +1 \\ \hline Lifetime & Stored Lifetime or PREQ Lifetime (whichever is longer) & Stored Lifetime or  PREQ Lifetime (whichever is longer) & Stored Lifetime or  PREP Lifetime (whichever is longer) & Stored Lifetime or  PREP Lifetime (whichever is longer) \\ \hline \end{tabular} Note: the initial value of the path metric is a constant value.  
\end{table} 
Table 3: Data for creating and updating forwarding information upon receiving PREQ and PREP [3]. The hop count refers to how many transmissions are required before a frame can reach its destination. Based on the drafted standards [3], if a direct transmission occurs without hopping, the hop count field will be recorded as 1. For HWMP, there is no need for a mesh STA to store information about all intermediate mesh STAs within a path. The hop count already reflects the number of mesh STAs in the path. Even though not all of the intermediate mesh STAs are known, the next hop mesh STA address must be identified. By referring to the routing table, a mesh STA can forward frames to the next hop address. An example of this operation is described in Figure 6 and Table 2. If mesh STA D attempts to send a packet to mesh STA A, it will transmit the packet to mesh STA C. Referring to its routing table, mesh STA C determines that the next hop for mesh STA A is mesh STA B; thus, mesh STA C can forward the packet to mesh STA B. The frame constructed by mesh STA C at this point will have transmitter address MAC C, source address MAC D, receiver address MAC B, and destination address MAC A. The forwarding information in the routing table will eventually time out.