Tor Padding Specification Mike Perry Note: This is an attempt to specify Tor as currently implemented. Future versions of Tor will implement improved algorithms. This document tries to cover how Tor chooses to use cover traffic to obscure various traffic patterns from external and internal observers. Other implementations MAY take other approaches, but implementors should be aware of the anonymity and load-balancing implications of their choices. THIS SPEC ISN'T DONE YET. 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. Overview Tor supports two classes of cover traffic: connection-level padding, and circuit-level padding. Connection-level padding uses the CELL_PADDING cell command for cover traffic, where as circuit-level padding uses the RELAY_COMMAND_DROP relay command. CELL_PADDING is single-hop only and can be differentiated from normal traffic by Tor relays ("internal" observers), but not by entities monitoring Tor OR connections ("external" observers). RELAY_COMMAND_DROP is multi-hop, and is not visible to intermediate Tor relays, because the relay command field is covered by circuit layer encryption. Moreover, Tor's 'recognized' field allows RELAY_COMMAND_DROP padding to be sent to any intermediate node in a circuit (as per Section 6.1 of tor-spec.txt). Currently, only single-hop CELL_PADDING is used by Tor. It is described in Section 2. At a later date, further sections will be added to this document to describe various uses of multi-hop circuit-level padding. 2. Connection-level padding 2.1. Background Tor clients and relays make use of CELL_PADDING to reduce the resolution of connection-level metadata retention by ISPs and surveillance infrastructure. Such metadata retention is implemented by Internet routers in the form of Netflow, jFlow, Netstream, or IPFIX records. These records are emitted by gateway routers in a raw form and then exported (often over plaintext) to a "collector" that either records them verbatim, or reduces their granularity further[1]. Netflow records and the associated data collection and retention tools are very configurable, and have many modes of operation, especially when configured to handle high throughput. However, at ISP scale, per-flow records are very likely to be employed, since they are the default, and also provide very high resolution in terms of endpoint activity, second only to full packet and/or header capture. Per-flow records record the endpoint connection 5-tuple, as well as the total number of bytes sent and received by that 5-tuple during a particular time period. They can store additional fields as well, but it is primarily timing and bytecount information that concern us. When configured to provide per-flow data, routers emit these raw flow records periodically for all active connections passing through them based on two parameters: the "active flow timeout" and the "inactive flow timeout". The "active flow timeout" causes the router to emit a new record periodically for every active TCP session that continuously sends data. The default active flow timeout for most routers is 30 minutes, meaning that a new record is created for every TCP session at least every 30 minutes, no matter what. This value can be configured from 1 minute to 60 minutes on major routers. The "inactive flow timeout" is used by routers to create a new record if a TCP session is inactive for some number of seconds. It allows routers to avoid the need to track a large number of idle connections in memory, and instead emit a separate record only when there is activity. This value ranges from 10 seconds to 600 seconds on common routers. It appears as though no routers support a value lower than 10 seconds. For reference, here are default values and ranges (in parenthesis when known) for common routers, along with citations to their manuals. Some routers speak other collection protocols than Netflow, and in the case of Juniper, use different timeouts for these protocols. Where this is known to happen, it has been noted. Inactive Timeout Active Timeout Cisco IOS[3] 15s (10-600s) 30min (1-60min) Cisco Catalyst[4] 5min 32min Juniper (jFlow)[5] 15s (10-600s) 30min (1-60min) Juniper (Netflow)[6,7] 60s (10-600s) 30min (1-30min) H3C (Netstream)[8] 60s (60-600s) 30min (1-60min) Fortinet[9] 15s 30min MicroTik[10] 15s 30min nProbe[14] 30s 120s Alcatel-Lucent[2] 15s (10-600s) 30min (1-600min) The combination of the active and inactive netflow record timeouts allow us to devise a low-cost padding defense that causes what would otherwise be split records to "collapse" at the router even before they are exported to the collector for storage. So long as a connection transmits data before the "inactive flow timeout" expires, then the router will continue to count the total bytes on that flow before finally emitting a record at the "active flow timeout". This means that for a minimal amount of padding that prevents the "inactive flow timeout" from expiring, it is possible to reduce the resolution of raw per-flow netflow data to the total amount of bytes send and received in a 30 minute window. This is a vast reduction in resolution for HTTP, IRC, XMPP, SSH, and other intermittent interactive traffic, especially when all user traffic in that time period is multiplexed over a single connection (as it is with Tor). 2.2. Implementation Tor clients currently maintain one TLS connection to their Guard node to carry actual application traffic, and make up to 3 additional connections to other nodes to retrieve directory information. We pad only the client's connection to the Guard node, and not any other connection. We treat Bridge node connections to the Tor network as client connections, and pad them, but otherwise not pad between normal relays. Both clients and Guards will maintain a timer for all application (ie: non-directory) TLS connections. Every time a non-padding packet is sent or received by either end, that endpoint will sample a timeout value from between 1.5 seconds and 9.5 seconds using the max(X,X) distribution described in Section 2.3. The time range is subject to consensus parameters as specified in Section 2.6. If the connection becomes active for any reason before this timer expires, the timer is reset to a new random value between 1.5 and 9.5 seconds. If the connection remains inactive until the timer expires, a single CELL_PADDING cell will be sent on that connection. In this way, the connection will only be padded in the event that it is idle, and will always transmit a packet before the minimum 10 second inactive timeout. 2.3. Padding Cell Timeout Distribution Statistics It turns out that because the padding is bidirectional, and because both endpoints are maintaining timers, this creates the situation where the time before sending a padding packet in either direction is actually min(client_timeout, server_timeout). If client_timeout and server_timeout are uniformly sampled, then the distribution of min(client_timeout,server_timeout) is no longer uniform, and the resulting average timeout (Exp[min(X,X)]) is much lower than the midpoint of the timeout range. To compensate for this, instead of sampling each endpoint timeout uniformly, we instead sample it from max(X,X), where X is uniformly distributed. If X is a random variable uniform from 0..R-1 (where R=high-low), then the random variable Y = max(X,X) has Prob(Y == i) = (2.0*i + 1)/(R*R). Then, when both sides apply timeouts sampled from Y, the resulting bidirectional padding packet rate is now a third random variable: Z = min(Y,Y). The distribution of Z is slightly bell-shaped, but mostly flat around the mean. It also turns out that Exp[Z] ~= Exp[X]. Here's a table of average values for each random variable: R Exp[X] Exp[Z] Exp[min(X,X)] Exp[Y=max(X,X)] 2000 999.5 1066 666.2 1332.8 3000 1499.5 1599.5 999.5 1999.5 5000 2499.5 2666 1666.2 3332.8 6000 2999.5 3199.5 1999.5 3999.5 7000 3499.5 3732.8 2332.8 4666.2 8000 3999.5 4266.2 2666.2 5332.8 10000 4999.5 5328 3332.8 6666.2 15000 7499.5 7995 4999.5 9999.5 20000 9900.5 10661 6666.2 13332.8 In this way, we maintain the property that the midpoint of the timeout range is the expected mean time before a padding packet is sent in either direction. 2.4. Maximum overhead bounds With the default parameters and the above distribution, we expect a padded connection to send one padding cell every 5.5 seconds. This averages to 103 bytes per second full duplex (~52 bytes/sec in each direction), assuming a 512 byte cell and 55 bytes of TLS+TCP+IP headers. For a client connection that remains otherwise idle for its expected ~50 minute lifespan (governed by the circuit available timeout plus a small additional connection timeout), this is about 154.5KB of overhead in each direction (309KB total). With 2.5M completely idle clients connected simultaneously, 52 bytes per second amounts to 130MB/second in each direction network-wide, which is roughly the current amount of Tor directory traffic[11]. Of course, our 2.5M daily users will neither be connected simultaneously, nor entirely idle, so we expect the actual overhead to be much lower than this. 2.5. Reducing or Disabling Padding via Negotiation To allow mobile clients to either disable or reduce their padding overhead, the CELL_PADDING_NEGOTIATE cell (tor-spec.txt section 7.2) may be sent from clients to relays. This cell is used to instruct relays to cease sending padding. If the client has opted to use reduced padding, it continues to send padding cells sampled from the range [9000,14000] milliseconds (subject to consensus parameter alteration as per Section 2.6), still using the Y=max(X,X) distribution. Since the padding is now unidirectional, the expected frequency of padding cells is now governed by the Y distribution above as opposed to Z. For a range of 5000ms, we can see that we expect to send a padding packet every 9000+3332.8 = 12332.8ms. We also half the circuit available timeout from ~50min down to ~25min, which causes the client's OR connections to be closed shortly there after when it is idle, thus reducing overhead. These two changes cause the padding overhead to go from 309KB per one-time-use Tor connection down to 69KB per one-time-use Tor connection. For continual usage, the maximum overhead goes from 103 bytes/sec down to 46 bytes/sec. If a client opts to completely disable padding, it sends a CELL_PADDING_NEGOTIATE to instruct the relay not to pad, and then does not send any further padding itself. 2.6. Consensus Parameters Governing Behavior Connection-level padding is controlled by the following consensus parameters: * nf_ito_low - The low end of the range to send padding when inactive, in ms. - Default: 1500 * nf_ito_high - The high end of the range to send padding, in ms. - Default: 9500 - If nf_ito_low == nf_ito_high == 0, padding will be disabled. * nf_ito_low_reduced - For reduced padding clients: the low end of the range to send padding when inactive, in ms. - Default: 9000 * nf_ito_high_reduced - For reduced padding clients: the high end of the range to send padding, in ms. - Default: 14000 * nf_conntimeout_clients - The number of seconds to keep circuits opened and available for clients to use. Note that the actual client timeout is randomized uniformly from this value to twice this value. This governs client OR conn lifespan. Reduced padding clients use half the consensus value. - Default: 1800 * nf_pad_before_usage - If set to 1, OR connections are padded before the client uses them for any application traffic. If 0, OR connections are not padded until application data begins. - Default: 1 * nf_pad_relays - If set to 1, we also pad inactive relay-to-relay connections - Default: 0 * nf_conntimeout_relays - The number of seconds that idle relay-to-relay connections are kept open. - Default: 3600 A. Acknowledgments This research was supported in part by NSF grants CNS-1111539, CNS-1314637, CNS-1526306, CNS-1619454, and CNS-1640548. 1. https://en.wikipedia.org/wiki/NetFlow 2. http://infodoc.alcatel-lucent.com/html/0_add-h-f/93-0073-10-01/7750_SR_OS_Router_Configuration_Guide/Cflowd-CLI.html 3. http://www.cisco.com/en/US/docs/ios/12_3t/netflow/command/reference/nfl_a1gt_ps5207_TSD_Products_Command_Reference_Chapter.html#wp1185203 4. http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/70974-netflow-catalyst6500.html#opconf 5. https://www.juniper.net/techpubs/software/erx/junose60/swconfig-routing-vol1/html/ip-jflow-stats-config4.html#560916 6. http://www.jnpr.net/techpubs/en_US/junos15.1/topics/reference/configuration-statement/flow-active-timeout-edit-forwarding-options-po.html 7. http://www.jnpr.net/techpubs/en_US/junos15.1/topics/reference/configuration-statement/flow-active-timeout-edit-forwarding-options-po.html 8. http://www.h3c.com/portal/Technical_Support___Documents/Technical_Documents/Switches/H3C_S9500_Series_Switches/Command/Command/H3C_S9500_CM-Release1648%5Bv1.24%5D-System_Volume/200901/624854_1285_0.htm#_Toc217704193 9. http://docs-legacy.fortinet.com/fgt/handbook/cli52_html/FortiOS%205.2%20CLI/config_system.23.046.html 10. http://wiki.mikrotik.com/wiki/Manual:IP/Traffic_Flow 11. https://metrics.torproject.org/dirbytes.html 12. http://freehaven.net/anonbib/cache/murdoch-pet2007.pdf 13. https://gitweb.torproject.org/torspec.git/tree/proposals/188-bridge-guards.txt 14. http://www.ntop.org/wp-content/uploads/2013/03/nProbe_UserGuide.pdf