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Glossary
The Tor Project
This document aims to specify terms, notations, and phrases related
to Tor, as used in the Tor specification documents and other documentation.
This glossary is not a design document; it is only a reference.
This glossary is a work-in-progress; double-check its definitions before
citing them authoritatively. ;)
Table of Contents
0. Preliminaries
1.0. Commonly used Tor configuration terms
2.0. Tor network components
2.1. Relays, aka OR (onion router)
2.1.1. Specific roles
2.2. Client, aka OP (onion proxy)
2.3. Authorities
2.4. Hidden Service
2.5. Circuit
2.6. Edge connection
2.7. Consensus
2.8. Descriptor
3.0. Tor network protocols
3.1. Link handshake
3.2. Circuit handshake
3.3. Hidden Service Protocol
3.4. Directory Protocol
4.0. General network definitions
0. Preliminaries
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
1.0. Commonly used Tor configuration terms
ORPort - Onion Router Port
DirPort - Directory Port
2.0. Tor network components
2.1. Relays, aka OR (onion router)
[Style guide: prefer the term "Relay"]
2.1.1. Specific roles
Exit relay: The final hop in an exit circuit before traffic leaves
the Tor network to connect to external servers.
Non-exit relay: Relays that send and receive traffic only to
other Tor relays and Tor clients.
Entry relay: The first hop in a Tor circuit. Can be either a guard
relay or a bridge, depending on the client's configuration.
Guard relay: A relay that a client uses as its entry for a longer
period of time. Guard relays are rotated more slowly to prevent
attacks that can come from being exposed to too many guards.
Bridge: A relay intentionally not listed in the public Tor
consensus, with the purpose of circumventing entities (such as
governments or ISPs) seeking to block clients from using Tor.
Currently, bridges are used only as entry relays.
Directory cache: A relay that downloads cached directory information
from the directory authorities and serves it to clients on demand.
Any relay will act as a directory cache, if its bandwidth is high enough.
Rendezvous point: A relay connecting a client to a hidden service.
Each party builds a three-hop circuit, meeting at the
rendezvous point.
2.2. Client, aka OP (onion proxy)
[Style: the "OP" and "onion proxy" terms are deprecated.]
2.3. Authorities:
Directory Authority: Nine total in the Tor network, operated by
trusted individuals. Directory authorities define and serve the
consensus document, defining the "state of the network." This document
contains a "router status" section for every relay currently
in the network. Directory authorities also serve router descriptors,
extra info documents, microdescriptors, and the microdescriptor consensus.
Bridge Authority: One total. Similar in responsibility to directory
authorities, but for bridges.
Fallback directory mirror: One of a list of directory caches distributed
with the Tor software. (When a client first connects to the network, and
has no directory information, it asks a fallback directory. From then on,
the client can ask any directory cache that's listed in the directory
information it has.)
2.4. Hidden Service:
A hidden service is a server that will only accept incoming
connections via the hidden service protocol. Connection
initiators will not be able to learn the IP address of the hidden
service, allowing the hidden service to receive incoming connections,
serve content, etc, while preserving its location anonymity.
2.5. Circuit:
An established path through the network, where cryptographic keys
are negotiated using the ntor protocol or TAP (Tor Authentication
Protocol (deprecated)) with each hop. Circuits can differ in length
depending on their purpose. See also Leaky Pipe Topology.
Origin Circuit -
Exit Circuit: A circuit which connects clients to destinations
outside the Tor network. For example, if a client wanted to visit
duckduckgo.com, this connection would require an exit circuit.
Internal Circuit: A circuit whose traffic never leaves the Tor
network. For example, a client could connect to a hidden service via
an internal circuit.
2.6. Edge connection:
2.7. Consensus: The state of the Tor network, published every hour,
decided by a vote from the network's directory authorities. Clients
fetch the consensus from directory authorities, fallback
directories, or directory caches.
2.8. Descriptor: Each descriptor represents information about one
relay in the Tor network. The descriptor includes the relay's IP
address, public keys, and other data. Relays send
descriptors to directory authorities, who vote and publish a
summary of them in the network consensus.
3.0. Tor network protocols
3.1. Link handshake
The link handshake establishes the TLS connection over which two
Tor participants will send Tor cells. This handshake also
authenticates the participants to each other, possibly using Tor
cells.
3.2. Circuit handshake
Circuit handshakes establish the hop-by-hop onion encryption
that clients use to tunnel their application traffic. The
client does a pairwise key establishment handshake with each
individual relay in the circuit. For every hop except the
first, these handshakes tunnel through existing hops in the
circuit. Each cell type in this protocol also has a newer
version (with a "2" suffix), e.g., CREATE2.
CREATE cell: First part of a handshake, sent by the initiator.
CREATED cell: Second part of a handshake, sent by the responder.
EXTEND cell: (also known as a RELAY_EXTEND cell) First part of a
handshake, tunneled through an existing circuit. The last relay
in the circuit so far will decrypt this cell and send the
payload in a CREATED cell to the chosen next hop relay.
EXTENDED cell: (also known as a RELAY_EXTENDED cell) Second part
of a handshake, tunneled through an existing circuit. The last
relay in the circuit so far receives the CREATED cell from the
new last hop relay and encrypts the payload in an EXTENDED cell
to tunnel back to the client.
Onion skin: A CREATE/CREATE2 or EXTEND/EXTEND2 payload that
contains the first part of the TAP or ntor key establishment
handshake.
3.3. Hidden Service Protocol
3.4. Directory Protocol
4.0. General network definitions
Leaky Pipe Topology: The ability for the origin of a circuit to address
relay cells to be addressed to any hop in the path of a circuit. In Tor,
the destination hop is determined by using the 'recognized' field of relay
cells.
Stream: A single application-level connection or request, multiplexed over
a Tor circuit. A 'Stream' can currently carry the contents of a TCP
connection, a DNS request, or a Tor directory request.
Channel: A pairwise connection between two Tor relays, or between a
client and a relay. Circuits are multiplexed over Channels. All
channels are currently implemented as TLS connections.
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