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When the gateway receives the access carrier from the node N 7 , it begins by decrypting
each secret information, from E G (k 1 ) to E G (k 6 ). By doing this, it discovers the
route that the carrier took and it can start decrypting each layer of the onioned request.
In the example above, the gateway peels off all onion’s layers using the obtained shared
keys and finds two plain requests: req 3 and req 6 .
3. Downlink Phase
The downlink phase takes place as soon as the gateway receives an access request.
When the gateway discovers a plain request, it obtains the desired Internet data on behalf
of the requester. After receiving the data, the gateway encapsulates it into a downlink
packet. This packet may contain data requested by different nodes. In order to delivery
the data requested, the gateway sends the downlink packet through an anonymous route.
This route is carefully chosen by the gateway to include, not only the requesters, but some
dummy nodes.
The downlink packet’s content is structured as an onion, similar to the
data carrier. Each onion’s layer may contain downlink data and the address
of the next hop in the route. The downlink packet has the following format:
E k1 (E k2 (E k3 (...E kl (G, down l , E G (nonce)), ...N 4 , down 3 ), N 3 , down 2 ), N 2 , down 1 ),
where down i is the downlink data destined to the requester N i and E G (nonce) is an
unique value included by the gateway for delivery confirmation purpose. Besides the
downlink data, down i also contains the reqId. It is intended to link the downlink traffic
with a given access request. Additionally, similar as in the data carrier, the gateway
pads down i with dummy bits in those layers where no data need to be included. In the
innermost layer of onion, we have the information destined to last node of the route N l ,
which should forward the packet to the gateway’s address (G).
The downlink protocol begins when the gateway sends the onion to the first node
in the route. Each mesh node decrypts one layer, checks for any downlink data, verifies
the next hop’s address and then forward the packet to it. Note that before forwarding, a
node N i removes both down i and N i+1 from its corresponding layer. Every mesh node
performs the same operation along the route, even if the down i is not a real downlink traffic.
This protocol continues until the packet reaches the last node in the route. This node
performs the operations described before and then forwards the packet to the gateway.
The packet’s content at this point is only E G (nonce). The gateway receives this data and
verifies that the packet visited every node of the route. Hence, this information works as
a delivery confirmation mechanism.
From the example presented in the access phase, suppose the gateway constructs
a downlink packet to delivery data to the requester nodes N 3 and N 6 . The dummy nodes
included in downlink route will be N 8 , N 9 , N 10 , and N 11 . The gateway makes an onion
to target the six nodes of the route in the following order: N 8 → N 9 → N 3 → N 10 →
N 6 → N 11 . The message flow, from the gateway until the node last node N 11 , is as
follows:
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