# 3 MSF: A Multi-Modal Adaptation of the 6 TiSCH Minimal Scheduling Function for the Industrial IoT
## 2 IPv6 over the TSCH Mode of IEEE 802.15.4
### 6 TiSCH Architecture Basics
#### 2.1.1 The Routing Layer  
RPL is 6T3 ECH's routing protocol. It functions as a distance-vector routing protocol, constructing a hierarchical/tree-shaped routing structure named a Destination-Oriented Directed Acyclic Graph (DODAG) that converges towards a root node. Within the DODAG, each non-root node (theoretically speaking) maintains three distinct neighbor sets, as depicted in Figure 2: (1) the candidate neighbor set; (2) the parent set; and (3) the preferred parent, which is the default next hop towards the root.  
These sets are managed through control messages known as DODAG Information Objects (DIOs). More specifically, a DIO advertises a node's rank, which is the distance-vector element of RPL, i.e., \"a scalar representation of the location or radius of a node within a DODAG Version\" ([PERSON], 2016) (p. 20). Following an end-to-end path to the DODAG root, node ranks are required to decrease in a monotonic fashion.  
Within a DODAG, the flow of traffic is primarily Multi-Point to Point (MP2P), making the mechanisms by which an upward routing topology is formed and maintained critical. For instance, when an RPL node wants to join a DODAG shortly after booting and receiving a DIO, it needs to attach itself to the DODAG first and acquire a rank before it may initiate self-advertising and potentially become a parent. A node is attached once it has a non-empty parent set and determines the rank to advertise (towards potential children) in forthcoming DIOs either during or after filtering the candidate neighbor set down to a suitable parent set and choosing a preferred parent from it.  
Discovering upward routes and forming/maintaining an upward routing topology are both controlled by an OF. As per RFC 6550 ([PERSON], 2016) (p. 67), \"the candidate neighbor set is a subset of the nodes that can be reached via link-local multicast. The selection of this set is implementation and OF-dependent\". Additionally, the OF largely dictates how a parent set should be formed from the set of candidate neighbors. That being said, OF-agnostic rules  
Figure 1: Example of a common implementation of the Internet Engineering Task Force (IETF) Internet Protocol version 6 (IPv6) over the Time-Slotted Channel Hopping (TSCH) mode of IEEE 802.15.4 (6T3 ECH) protocol stack. Network-layer functionality is indicated in red, whereas link-layer functionality is marked blue. Reprinted from [1] with permission from the authors.  
Figure 2: Connection between the three Routing Protocol for Low-power and Lossy Networks (LLNs) (RPL) neighbor sets maintained by each non-root node. The formation of these sets is prescribed by an RPL Objective Function (OF). Reprinted from [8] with permission from the authors.  
do apply. The primary tasks of the OF are to select a preferred parent and compute a rank to advertise. Both usually involve a routing metric [10]. Whether or not rank computation is part of parent set formation and preferred parent selection can differ between OFs. We kindly refer the reader to prior work [8] for a more detailed explanation of RPL and the role of OFs.