Module padding

Channel padding

Tor spec padding-spec.txt section 2.

Overview of channel padding control arrangements

1. tor_chanmgr::mgr::map collates information about dormancy, netdir, and overall client configuration, to maintain a ChannelPaddingInstructions which is to be used for all relevant¹ channels. This is distributed to channel frontends (Channels) by calling Channel::reparameterize.

2. Circuit and channel get_or_launch methods all take a ChannelUsage. This is plumbed through the layers to AbstractChanMgr::get_or_launch, which passes it to the channel frontend via Channel::note_usage.

3. The Channel collates this information, and maintains an idea of whether padding is relevant for this channel (PaddingControlState). For channels where it is relevant, it sends CtrlMsg::ConfigUpdate to the reactor.

4. The reactor handles CtrlMsg::ConfigUpdate by reconfiguring is padding timer; and by sending PADDING_NEGOTIATE cell(s).

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1. A “relevant” channel is one which is not excluded by the rules about padding in padding-spec 2.2. Arti does not currently support acting as a relay, so all our channels are client-to-guard or client-to-directory. ↩

Structs

PaddingTimerProj 🔒

Parameters

Timing parameters, as described in padding-spec.txt

ParametersBuilder

Builder for Parameters.

PreparedParameters 🔒

Timing parameters, “compiled” into a form which can be sampled more efficiently

Timer 🔒

Timer that organises wakeups when channel padding should be sent

Enums

ParametersError

An error that occurred whil e constructing padding parameters.

SleepInstructions 🔒

Return value from prepare_to_sleep: instructions for what caller ought to do