Expand description

Coding and decoding for the cell types that make up Tor’s protocol


Tor’s primary network protocol is oriented around a set of messages called “Cells”. They exist at two primary layers of the protocol: the channel-cell layer, and the relay-cell layer.

Channel cells are sent between relays, or between a client and a relay, over a TLS connection. Each of them encodes a single Channel Message. Channel messages can affect the channel itself (such as those used to negotiate and authenticate the channel), but more frequently are used with respect to a given multi-hop circuit.

Channel message that refer to a circuit do so with a channel-local identifier called a Circuit ID. These messages include CREATE2 (used to extend a circuit to a first hop) and DESTROY (used to tear down a circuit). But the most frequently used channel message is RELAY, which is used to send a message to a given hop along a circuit.

Each RELAY cell is encrypted and decrypted (according to protocols not implemented in this crate) until it reaches its target. When it does, it is decoded into a single Relay Message. Some of these relay messages are used to manipulate circuits (e.g., by extending the circuit to a new hop); others are used to manipulate anonymous data-streams (by creating them, ending them, or sending data); and still others are used for protocol-specific purposes (like negotiating with an onion service.)

For a list of most of the cell types used in Tor, see tor-spec.txt. Other cell types are defined in rend-spec-v3.txt (for onion services) and padding-spec.txt (for padding negotiation).

This crate is part of Arti, a project to implement Tor in Rust.

Futureproofing note

There are two pending proposals to remove the one-to-one correspondence between relay cells and relay messages.

Proposal 319 would add a “RELAY_FRAGMENT” command that would allow larger relay messages to span multiple RELAY cells.

Proposal 325, on the other hand, would allow multiple relay messages to be packed into a single RELAY cell.

The distinction between RelayCell and RelayMsg is meant in part to future-proof arti against these proposals if they are adopted.


Messages sent over Tor channels

Implementation for parsing and encoding relay cells


An error type for the tor-cell crate.

Type Definitions

An error type for this crate.