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+Filename: 171-separate-streams-by-port-or-host.txt
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+Title: Separate streams across circuits by destination port or destination host
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+Author: Robert Hogan, Jacob Appelbaum, Damon McCoy
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+Created: 21-Oct-2008
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+Modified: 30-Aug-2010
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+Status: Draft
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+
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+Motivation:
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+
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+Streams are currently attached to circuits without regard to their content,
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+destination host, or destination port. We propose three options,
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+IsolateBySOCKSUser, IsolateStreamsByPort and IsolateStreamsByHost to change the
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+default behavior.
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+
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+The contents of some streams will always have revealing plain text information;
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+these streams should be treated differently than other streams that may or may
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+not have unencrypted PII content. DNS, with the exception of DNSCurve, is
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+always unencrypted. It is reasonable to assume that other protocols may exist
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+that have a similar issue and may cause user concern. It is also the case that
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+we must balance network load issues and stream privacy. The Tor network will not
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+currently scale to one circuit per application connection nor should it anytime
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+soon.
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+
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+Circuits are currently created with a few constraints and are rotated within
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+a reasonable time window. This allows a rogue exit node to correlate all
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+streams on a given circuit.
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+
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+Design:
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+
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+We propose two options for isolation of streams that lessen the observability
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+and linkability of the Tor client's traffic.
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+
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+IsolateStreamsByPort will take a list of ports or optionally the keyword 'All'
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+in place of a port list. The use of the keyword 'All' will ensure that all
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+application connections attached to streams will be isolated to separate
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+circuits by port number.
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+
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+IsolateStreamsByHost will take a boolean value. When enabled, all application
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+connections, regardless of port number will be isolated with separate circuits
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+per host. If this option is enabled, we should ensure that the client has a
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+reasonable number of pre-built circuits to ensure perceived performance. This
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+should also intentionally limit the total number of circuits a client will
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+build to ten circuits to prevent abuse and load on the network. This is a
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+trade-off of performance for anonymity. Tor will issue a warning if a client
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+encounters this limit.
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+
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+IsolateBySOCKSUser will take a boolean value. When enabled, all application
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+connections, regardless of port number will be isolated with separate circuits
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+per SOCKS username. This options ensures that any two streams that were created
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+with different SOCKS usernames will be sent over different circuits. The empty
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+username will be treated as its own username different from all other usernames.
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+
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+Security implications:
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+
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+It is believed that the proposed changes will improve the anonymity for end
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+user stream privacy. The end user will no longer link all streams at a single
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+exit node during a given time window.
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+
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+There is a possible attack where a hostile web page possibly in collusion with
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+an exit node contains image links for images at (say) "evil.example.com:53" and
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+"evil.example.com:31337", and thereby (if they're lucky) correlate port-80
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+circuits with port-53 and port-31337 circuits.
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+
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+Specification:
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+
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+The Tor client circuit selection process is not entirely specified. Any client
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+circuit specification must take these changes into account.
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+
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+Compatibility:
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+
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+The proposed changes should not create any compatibility issues. New Tor clients
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+will be able to take advantage of this without any modification to the network.
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+
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+Implementation:
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+
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+It is further proposed that IsolateStreamsByPort will be enabled by default
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+for port 22, 53, and port 80.
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+
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+It is further proposed that IsolateStreamsByHost will be disabled by default.
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+
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+Implementation notes:
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+
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+The implementation of this option may want to consider cases where the same
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+exit node is shared by two or more circuits and IsolateStreamsByPort is in
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+force. Since the purpose of the option is to reduce the opportunity of Exit
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+Nodes to attack traffic from the same source on multiple ports, the
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+implementation may need to ensure that circuits reserved for the exclusive use
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+of given ports do not share the same exit node.
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+
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+Circuits should not be shared by unique clients. Tor should check to ensure
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+that peer IP addresses are identical when they connect to the SOCKS listener or
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+the TransPort listener before sharing a circuit. If the addresses are not
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+identical, Tor should ensure that the circuits are not shared.
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+
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+Performance and scalability notes:
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+
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+It is further proposed that IsolateStreamsByPort will be enabled by default for
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+all ports after a reasonable assessment is performed. Specifically, we should
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+determine the impact this option has on Tor clients and the Tor network.
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Jacob Appelbaum