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<a id="padding-spec.txt-1"></a>
# Overview
Tor supports two classes of cover traffic: connection-level padding, and
circuit-level padding.
Connection-level padding uses the CELL_PADDING cell command for cover
traffic, where as circuit-level padding uses the RELAY_COMMAND_DROP relay
command. CELL_PADDING is single-hop only and can be differentiated from
normal traffic by Tor relays ("internal" observers), but not by entities
monitoring Tor OR connections ("external" observers).
RELAY_COMMAND_DROP is multi-hop, and is not visible to intermediate Tor
relays, because the relay command field is covered by circuit layer
encryption. Moreover, Tor's 'recognized' field allows RELAY_COMMAND_DROP
padding to be sent to any intermediate node in a circuit (as per Section
6.1 of tor-spec.txt).
Tor uses both connection level and circuit level padding. Connection
level padding is described in section 2. Circuit level padding is
described in section 3.
The circuit-level padding system is completely orthogonal to the
connection-level padding. The connection-level padding system regards
circuit-level padding as normal data traffic, and hence the connection-level
padding system will not add any additional overhead while the circuit-level
padding system is actively padding.
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