2.1 Link Quality

The link quality between a directed node pair (A,B) is the probability that a packet transmitted by A will be successfully received by B. The bidirectional link quality of an undirected node pair (A,B) is the product of the link quality of (A,B) and (B,A). This definition assumes independent link losses. It also includes the case when the link quality of (A,B) and (B,A) are different; this can occur due to local interference or noise.

2.2 In-bound Link Quality

In a node pair (A,B), with B as the node of reference, in-bound link quality is a value in the range of 0 to 255 that describes the quality of the link from A to B estimated by B by counting the successfully received packets from A among all the transmitted packets or using link quality indicators such as LQI and RSSI provided by the radio on the node B, or some other methods.

2.3 Out-bound Link Quality

In a node pair (A,B), with B as the node of reference, out-bound link quality is defined as the quality of the link from B to A. B can determine the out-bound link quality if A advertises its in-bound link qualities. LEEP is the protocol that is used to exchange the in-bound link qualities.

2.4 Link Information Entry

Link Information Entry created by node k is a tuple (n,q) where q is the in-bound link quality from node n to k.

3.1 Assumptions

Following are the assumptions made by LEEP:

3.1.1. The data link frame has a single-hop source field. 3.1.2. The data link layer provides a broadcast address. 3.1.3. The data link layer provides the length of the LEEP frame.

3.2 The Protocol

To compute the bi-directional link quality, in-bound link quality must be exchanged among the neighbors. LEEP maintains a sequence number that is incremented by one for each outgoing LEEP frame. The sequence number in the LEEP frame MUST be incremented by one even if the data link layer retransmits the LEEP frame. The LEEP sequence number MAY be used to count the number of missing packets to estimate the in-bound link quality from the transmitter. LEEP MUST transmit Link Information entries describing the in-bound link qualities for a subset of its neighbors. The Link Information entry on the LEEP frame allows the receiver node to find the out-bound link quality to the transmitter node identified by the data link source address.

3.3 LEEP Frame

A LEEP frame has a header, the payload, and a footer with the Link Information (LI) entries as shown in this diagram:

-------------------------------------------------------------
|   LEEP   | Payload | LI Entry | LI Entry | ... | LI Entry |
|  Header  |         |     1    |     2    |     |     n    |
-------------------------------------------------------------

The number of Link Information entries can be different in each outgoing LEEP frame. The number of Link Information entries MUST not increase the size of the LEEP frame beyond the maximum payload length allowed by the data link layer. A LEEP frame can have 0 Link Information entry.

3.3.1 LEEP header

The following diagram shows the LEEP header format:

                     1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|nentry | rsrvd |      seqno      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Field definitions:

• nentry - Number of Link Information entries in the footer
• seqno - LEEP sequence number.
• rsrvd - Reserved and must be set to 0.

3.3.2 Link Information Entry

The following diagram shows the Link Information Entry format:

                     1
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             node id             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| link quality  |
+-+-+-+-+-+-+-+-+

Field definitions:

• node id: the link layer address of the neighbor
• link quality: The in-bound link quality from the node identified by node id to the node that transmits this Link Information entry