17. Glossary


17.1. From relays’ configuration


A token bucket limits the average incoming/outgoing bandwidth (default 1GB)
Limit the maximum token bucket size (also known as the burst) in each direction (default: 1GB)
will not advertise more than this amount of bandwidth for our BandwidthRate Server operators who want to reduce the number of clients who ask: to build circuits through them (since this is proportional to advertised bandwidth rate) can thus reduce the CPU demands on their server without impacting network performance.

17.2. From descriptors


average bandwidth:

bw that the relay is willing to sustain over long periods

bandwidth-avg = min(RelayBandwidthRate, BandwidthRate, MaxAdvertisedBandwidth)
burst bandwidth:

bw that the relay is willing to sustain in very short intervals

bandwidth-burst = min(RelayBandwidthBust, BandwidthBurst)
observed bandwidth:
min(max bandwidth sustained output over any 10secs in the past day, another sustained input)
advertised bandwidth:
min(average bandwidth, observed bandwidth, 10MB/s)

See Bandwidth values in relays’ descriptor

17.3. From consensus


advertised bandwidth
= min(average bandwidth, observed bandwidth, 10MB/s)) KB/s

If 3 or more authorities provide a Measured= keyword for a router, the authorities produce a consensus containing a “w” Bandwidth= keyword equal to the median of the Measured= votes.

Bandwidth = observed bandwidth * (stream bandwidth / stream mu)

See Bandwidth values in dirauths’ consensus documents

17.4. From metrics in rst


advertised bandwidth:
the volume of traffic, both incoming and outgoing, that a relay is willing to sustain, as configured by the operator and claimed to be observed from recent data transfers. [DIRSPEC427]
bandwidth history:
the volume of incoming and/or outgoing traffic that a relay claims to have handled on behalf of clients. [DIRSPEC833]
a relay whose existence is non-public and which can therefore provide access for blocked clients, often in combination with pluggable transports, which registers itself with the bridge authority.
bridge authority:
a special-purpose relay that maintains a list of bridges as input for external bridge distribution mechanisms (for example, BridgeDB).
a path through the Tor network built by clients starting with a bridge or relay and optionally continued by additional relays to hide the source of the circuit.
a node in the Tor network, typically running on behalf of one user, that routes application connections over a series of relays.
a single document compiled and voted on by the directory authorities once per hour, ensuring that all clients have the same information about the relays that make up the Tor network.
consensus weight:
a value assigned to a relay that is based on bandwidth observed by the relay and bandwidth measured by the directory authorities, included in the hourly published consensus, and used by clients to select relays for their circuits. [DIRSPEC2337]
directory authority:
a special-purpose relay that maintains a list of currently-running relays and periodically publishes a consensus together with the other directory authorities.
directory mirror:
a relay that provides a recent copy of directory information to clients, in order to reduce the load on directory authorities.
onion service:
a service (for example, a website or instant-messaging server) that is only accessible via the Tor network.
pluggable transport:
an alternative transport protocol provided by bridges and used by clients to circumvent transport-level blockings (for example, by ISPs or governments).
a publicly-listed node in the Tor network that forwards traffic on behalf of clients, and that registers itself with the directory authorities.
relay flag:
a special (dis-)qualification of relays for circuit positions (for example, “Guard”, “Exit”, “BadExit”), circuit properties (for example, “Fast”, “Stable”), or roles (for example, “Authority”, “HSDir”), as assigned by the directory authorities and further defined in the directory protocol specification.

17.5. From tor code in rst


Onion Router Port
Directory Port
aka OR (onion router)
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.
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.
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.)
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.
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.
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.

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.

scriptor: ach descriptor represents information about one elay in the Tor network. The descriptor includes the relay’s IP ddress, public keys, and other data. Relays send escriptors to directory authorities, who vote and publish a ummary of them in the network consensus.

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.
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.
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.
(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.
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.
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.
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.