7, min 7,round 1
3.3 AODV-MPLS Protocol
3.3.4 Forward Equivalence Classes
In the opposite direction however, the destination stores a running average of the label values for each node in the network to determine exactly how long it takes for a packet to traverse a path to reach the intended source node. Because multiple paths exist between each source node and the destination, and the same paths can be used in the forward and reverse direction, the destination will store a running average of the past four labels for a given node that uses distinct paths between the source and destination.
Octets: 1 1 2 2 1 8 8
Figure 3. 14 Zigbee Route Reply Command Frame
Bits: 8 8 64 64 8 20 24
Figure 3. 15 AODV-MPLS Route Reply Command Frame
3.3.4 Forward Equivalence Classes
The premise of this protocol is the FEC, and the mapping of labels to a particular FEC.
A FEC consists of a group of packets, each of which is forwarded along the same
addresses of routers in a network, the FEC in our protocol is based on the timing information described above. MPLS provides a form of Quality of Service, whereby each node in the network will guarantee that only a certain amount of time will be spent processing and forwarding the packet towards its destination. Applications that currently utilize this concept are: Voice over IP (VoIP), online gaming, as well as IP-TV. Although the applications considered herein do not require the same type of service, larger networks with stringent time requirements would benefit from a similar approach.
A node that generates a packet is referred to as an ingress node; the node for which this packet is destined is called the egress node. The nodes between these two nodes are called label-shifting nodes. They perform a label shifting operation to ensure that the packet is routed along the correct path to the destination in the forward direction, and to the correct source node in the reverse direction. A connection established between a source node (ingress node) and the destination node (egress node) is referred to as a label shifted path. Thus each ingress node, selects a path with an estimated latency that is less than or equal to the amount of latency it is willing to tolerate. The ingress node then places all packets with similar latency requirements into the same FEC. This is equivalent to saying that all packets that traverse the same path with a latency,
X
i represents the ith Forward Equivalence Class that the packets!
The index k represents the number of packets that belong to any given FEC, thus the number of packets from FEC to FEC need not be the same.
3.3.5 Labels
A label is a fixed length packet identifier used between two adjacent nodes. The label assigned to a particular packet represents the Forward Equivalence Class that the packet belongs to. The label not only uniquely identifies the FEC that a packet belongs to, but it also uniquely identifies which neighboring node the packet came from.
Labels drive the routing decisions that are made at each node. Because the label uniquely identifies the neighbor from whence a packet comes from, as well as the FEC that it belongs to, the recipient of a packet need only look at the label to determine which neighbor sent the packet. The recipient then determines the next hop for the packet based on the FEC that the packet belongs to, which is determined by the label attached to the packet. The node removes the label from the packet, and then attaches another label to the packet corresponding to the next hop in the FEC. Then the node forwards the packet to the next hop and the process continues until it reaches its destination. This tremendously reduces the amount of time spent processing each data packet. Figure 3.9 illustrates the general network frame format. The frame can have as little as
!
64 + n
bits where n represents the variable size of the payload. This is only possible if the: IEEE source and destination addresses, the source route subframe, and multicast control subframe fields are removed. Conversely the frame can include as many as!
136 + 16(m + 1) + n bits with the first term accounting for all terms excluding the variable payload and source route subframe fields. Both of which are accounted for by the variables n and m respectively. The additional 16 bits in the second term represent the control information included in the source route subframe field as illustrated in figure 3.16.
Octets: 1 1 Variable
Relay Count Relay Index Relay List
Figure 3. 16 Source Route Subframe
As will be explained in Section F, source routing is one of two options that have to be used in order to route packets from destination to the sink if a label shifting technique is not used. As a result this drastically increases the amount of unnecessary
information needed in order to route a message from the destination to a source node.
With a label solution each node only reads the 24-bit label that is attached to the leftmost field of the network header to determine the next hop in the FEC. The label corresponding to the node in the next hop list of the FEC is attached to the packet and then retransmitted. This reduces the overhead associated with packet processing, thereby decreasing the end-to-end latency.