Extreme Opportunistic Routing (ExOR) [1] is a
state-of-the-art OR protocol for wireless multihop
networks and has been implemented on the
RoofNet testbed at Massachusetts Institute of
Technology (MIT). ExOR integrates routing and
MAC protocols. It improves routing performance
by utilizing long-range but lossy links.
ExOR is designed for batch forwarding. The
source node includes a forwarder list in each
packet, prioritized by ETX distance to the destination:
the shorter the distance, the higher the
priority. Only those nodes that are closer to the
destination than the source are included in the
forwarder set. Each packet has a BITMAP
option, which marks those packets that have
been received by the sending node or nodes with
higher priorities. All packets are broadcast. A
forwarder transmits a packet only if no forwarder
with higher priority has explicitly
acknowledged receipt of it, as indicated in the
BITMAP position for this packet.
ExOR has good routing performance. However,
it also has the following drawbacks. First, it
reduces spatial reuse because it enforces global
coordination among forwarders. Second, forwarders
connected with low-quality links or no
links can make inconsistent decisions on packet
forwarding because a forwarder may not hear
the acknowledgment from other forwarders with
higher priorities, which causes duplicate transmissions.