Topology

The mixnodes for a session should be fully connected. That is, all mixnodes should attempt to maintain connections to all other mixnodes. The connections should be maintained for the whole session, as well as for the first 3 phases of the following session.

Mixnodes should also accept connections from non-mixnodes. Non-mixnodes should attempt to connect to a small number of "gateway" mixnodes in each active session.

Route generation

Routes through the mixnet are always generated in the context of a session, for a single packet/SURB. If multiple packets are to be sent, or multiple SURBs are to be built, a separate route should be generated for each one.

There are three kinds of route that a node may need to generate:

• From the node to a mixnode. For requests and drop cover traffic.
• From a mixnode to the node. For SURBs (sent in requests to enable replies).
• From the node to itself. For loop cover traffic.

Routes should be no longer than 7 nodes, including the source and destination nodes. The intermediate mixnodes should be uniformly randomly chosen, subject to the following constraints:

• If the generating node is not a mixnode, any nodes immediately preceding/following it must be connected gateway mixnodes.
• No mixnode should appear in a route more than once, unless this is unavoidable. For example, when a non-mixnode is generating a loop route, if there is only a single connected gateway mixnode, the gateway must necessarily appear twice.
• No node should ever appear twice consecutively in a route.

Note that although very short routes are possible, longer routes provide more anonymity. As such, it is recommended that by default nodes generate the longest routes possible (7 nodes).