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diff --git a/attic/text_formats/padding-spec.txt b/attic/text_formats/padding-spec.txt new file mode 100644 index 0000000..206a7f1 --- /dev/null +++ b/attic/text_formats/padding-spec.txt @@ -0,0 +1,625 @@ + + Tor Padding Specification + + Mike Perry, George Kadianakis + +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. + + 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. + +Table of Contents + + 1. Overview + 2. Connection-level padding + 2.1. Background + 2.2. Implementation + 2.3. Padding Cell Timeout Distribution Statistics + 2.4. Maximum overhead bounds + 2.5. Reducing or Disabling Padding via Negotiation + 2.6. Consensus Parameters Governing Behavior + 3. Circuit-level padding + 3.1. Circuit Padding Negotiation + 3.2. Circuit Padding Machine Message Management + 3.3. Obfuscating client-side onion service circuit setup + 3.3.1. Common general circuit construction sequences + 3.3.2. Client-side onion service introduction circuit obfuscation + 3.3.3. Client-side rendezvous circuit hiding + 3.3.4. Circuit setup machine overhead + 3.4. Circuit padding consensus parameters + A. Acknowledgments + +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). + + Tor uses both connection level and circuit level padding. Connection + level padding is described in section 2. Circuit level padding is + described in section 3. + + The circuit-level padding system is completely orthogonal to the + connection-level padding. The connection-level padding system regards + circuit-level padding as normal data traffic, and hence the connection-level + padding system will not add any additional overhead while the circuit-level + padding system is actively 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). + + Though flow measurement in principle can be bidirectional (counting cells + sent in both directions between a pair of IPs) or unidirectional (counting + only cells sent from one IP to another), we assume for safety that all + measurement is unidirectional, and so traffic must be sent by both parties + in order to prevent record splitting. + +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 padding packet sent by an + endpoint, that endpoint will sample a timeout value from + the max(X,X) distribution described in Section 2.3. The default + range is from 1.5 seconds to 9.5 seconds time range, subject to consensus + parameters as specified in Section 2.6. + + (The timing is randomized to avoid making it obvious which cells are + padding.) + + If another cell is sent for any reason before this timer expires, the timer + is reset to a new random value. + + If the connection remains inactive until the timer expires, a + single CELL_PADDING cell will be sent on that connection (which will + also start a new timer). + + In this way, the connection will only be padded in a given direction in + the event that it is idle in that direction, and will always transmit a + packet before the minimum 10 second inactive timeout. + + (In practice, an implementation may not be able to determine when, + exactly, a cell is sent on a given channel. For example, even though the + cell has been given to the kernel via a call to `send(2)`, the kernel may + still be buffering that cell. In cases such as these, implementations + should use a reasonable proxy for the time at which a cell is sent: for + example, when the cell is queued. If this strategy is used, + implementations should try to observe the innermost (closest to the wire) + queue that they practically can, and if this queue is already nonempty, + padding should not be scheduled until after the queue does become empty.) + +2.3. Padding Cell Timeout Distribution Statistics + + To limit the amount of padding sent, 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 + + +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. + + Currently, clients negotiate padding only when a channel is created, + immediately after sending their NETINFO cell. Recipients SHOULD, however, + accept padding negotiation messages at any time. + + If a client which previously negotiated reduced, or disabled, padding, and + wishes to re-enable default padding (ie padding according to the consensus + parameters), it SHOULD send CELL_PADDING_NEGOTIATE START with zero in the + ito_low_ms and ito_high_ms fields. (It therefore SHOULD NOT copy the values + from its own established consensus into the CELL_PADDING_NEGOTIATE cell.) + This avoids the client needing to send updated padding negotiations if the + consensus parameters should change. The recipient's clamping of the timing + parameters will cause the recipient to use its notion of the consensus + parameters. + + Clients and bridges MUST reject padding negotiation messages from relays, + and close the channel if they receive one. + +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 never-used circuits opened and + available for clients to use. Note that the actual client timeout is + randomized uniformly from this value to twice this value. + - The number of seconds to keep idle (not currently used) canonical + channels are open and available. (We do this to ensure a sufficient + time duration of padding, which is the ultimate goal.) + - This value is also used to determine how long, after a port has been + used, we should attempt to keep building predicted circuits for that + port. (See path-spec.txt section 2.1.1.) This behavior was + originally added to work around implementation limitations, but it + serves as a reasonable default regardless of implementation. + - For all use cases, reduced padding clients use half the consensus + value. + - Implementations MAY mark circuits held open past the reduced padding + quantity (half the consensus value) as "not to be used for streams", + to prevent their use from becoming a distinguisher. + - 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 + + +3. Circuit-level padding + + The circuit padding system in Tor is an extension of the WTF-PAD + event-driven state machine design[15]. At a high level, this design places + one or more padding state machines at the client, and one or more padding + state machines at a relay, on each circuit. + + State transition and histogram generation has been generalized to be fully + programmable, and probability distribution support was added to support more + compact representations like APE[16]. Additionally, packet count limits, + rate limiting, and circuit application conditions have been added. + + At present, Tor uses this system to deploy two pairs of circuit padding + machines, to obscure differences between the setup phase of client-side + onion service circuits, up to the first 10 cells. + + This specification covers only the resulting behavior of these padding + machines, and thus does not cover the state machine implementation details or + operation. For full details on using the circuit padding system to develop + future padding defenses, see the research developer documentation[17]. + +3.1. Circuit Padding Negotiation + + Circuit padding machines are advertised as "Padding" subprotocol versions + (see tor-spec.txt Section 9). The onion service circuit padding machines are + advertised as "Padding=2". + + Because circuit padding machines only become active at certain points in + circuit lifetime, and because more than one padding machine may be active at + any given point in circuit lifetime, there is also a padding negotiation + cell and a negotiated response. These are relay commands 41 and 42, with + relay headers as per section 6.1 of tor-spec.txt. + + The fields of the relay cell Data payload of a negotiate request are + as follows: + + const CIRCPAD_COMMAND_STOP = 1; + const CIRCPAD_COMMAND_START = 2; + + const CIRCPAD_RESPONSE_OK = 1; + const CIRCPAD_RESPONSE_ERR = 2; + + const CIRCPAD_MACHINE_CIRC_SETUP = 1; + + struct circpad_negotiate { + u8 version IN [0]; + u8 command IN [CIRCPAD_COMMAND_START, CIRCPAD_COMMAND_STOP]; + + u8 machine_type IN [CIRCPAD_MACHINE_CIRC_SETUP]; + + u8 unused; // Formerly echo_request + + u32 machine_ctr; + }; + + When a client wants to start a circuit padding machine, it first checks that + the desired destination hop advertises the appropriate subprotocol version for + that machine. It then sends a circpad_negotiate cell to that hop with + command=CIRCPAD_COMMAND_START, and machine_type=CIRCPAD_MACHINE_CIRC_SETUP (for + the circ setup machine, the destination hop is the second hop in the + circuit). The machine_ctr is the count of which machine instance this is on + the circuit. It is used to disambiguate shutdown requests. + + When a relay receives a circpad_negotiate cell, it checks that it supports + the requested machine, and sends a circpad_negotiated cell, which is formatted + in the data payload of a relay cell with command number 42 (see tor-spec.txt + section 6.1), as follows: + + struct circpad_negotiated { + u8 version IN [0]; + u8 command IN [CIRCPAD_COMMAND_START, CIRCPAD_COMMAND_STOP]; + u8 response IN [CIRCPAD_RESPONSE_OK, CIRCPAD_RESPONSE_ERR]; + + u8 machine_type IN [CIRCPAD_MACHINE_CIRC_SETUP]; + + u32 machine_ctr; + }; + + If the machine is supported, the response field will contain + CIRCPAD_RESPONSE_OK. If it is not, it will contain CIRCPAD_RESPONSE_ERR. + + Either side may send a CIRCPAD_COMMAND_STOP to shut down the padding machines + (clients MUST only send circpad_negotiate, and relays MUST only send + circpad_negotiated for this purpose). + + If the machine_ctr does not match the current machine instance count + on the circuit, the command is ignored. + +3.2. Circuit Padding Machine Message Management + + Clients MAY send padding cells towards the relay before receiving the + circpad_negotiated response, to allow for outbound cover traffic before + negotiation completes. + + Clients MAY send another circpad_negotiate cell before receiving the + circpad_negotiated response, to allow for rapid machine changes. + + Relays MUST NOT send padding cells or circpad_negotiated cells, unless a + padding machine is active. Any padding-related cells that arrive at the client + from unexpected relay sources are protocol violations, and clients MAY + immediately tear down such circuits to avoid side channel risk. + +3.3. Obfuscating client-side onion service circuit setup + + The circuit padding currently deployed in Tor attempts to hide client-side + onion service circuit setup. Service-side setup is not covered, because doing + so would involve significantly more overhead, and/or require interaction with + the application layer. + + The approach taken aims to make client-side introduction and rendezvous + circuits match the cell direction sequence and cell count of 3 hop general + circuits used for normal web traffic, for the first 10 cells only. The + lifespan of introduction circuits is also made to match the lifespan + of general circuits. + + Note that inter-arrival timing is not obfuscated by this defense. + +3.3.1. Common general circuit construction sequences + + Most general Tor circuits used to surf the web or download directory + information start with the following 6-cell relay cell sequence (cells + surrounded in [brackets] are outgoing, the others are incoming): + + [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [BEGIN] -> CONNECTED + + When this is done, the client has established a 3-hop circuit and also opened + a stream to the other end. Usually after this comes a series of DATA cell that + either fetches pages, establishes an SSL connection or fetches directory + information: + + [DATA] -> [DATA] -> DATA -> DATA...(inbound cells continue) + + The above stream of 10 relay cells defines the grand majority of general + circuits that come out of Tor browser during our testing, and it's what we use + to make introduction and rendezvous circuits blend in. + + Please note that in this section we only investigate relay cells and not + connection-level cells like CREATE/CREATED or AUTHENTICATE/etc. that are used + during the link-layer handshake. The rationale is that connection-level cells + depend on the type of guard used and are not an effective fingerprint for a + network/guard-level adversary. + +3.3.2. Client-side onion service introduction circuit obfuscation + + Two circuit padding machines work to hide client-side introduction circuits: + one machine at the origin, and one machine at the second hop of the circuit. + Each machine sends padding towards the other. The padding from the origin-side + machine terminates at the second hop and does not get forwarded to the actual + introduction point. + + From Section 3.3.1 above, most general circuits have the following initial + relay cell sequence (outgoing cells marked in [brackets]): + + [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [BEGIN] -> CONNECTED + -> [DATA] -> [DATA] -> DATA -> DATA...(inbound data cells continue) + + Whereas normal introduction circuits usually look like: + + [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 + -> [INTRO1] -> INTRODUCE_ACK + + This means that up to the sixth cell (first line of each sequence above), + both general and intro circuits have identical cell sequences. After that + we want to mimic the second line sequence of + + -> [DATA] -> [DATA] -> DATA -> DATA...(inbound data cells continue) + + We achieve this by starting padding INTRODUCE1 has been sent. With padding + negotiation cells, in the common case of the second line looks like: + + -> [INTRO1] -> [PADDING_NEGOTIATE] -> PADDING_NEGOTIATED -> INTRO_ACK + + Then, the middle node will send between INTRO_MACHINE_MINIMUM_PADDING (7) and + INTRO_MACHINE_MAXIMUM_PADDING (10) cells, to match the "...(inbound data cells + continue)" portion of the trace (aka the rest of an HTTPS response body). + + We also set a special flag which keeps the circuit open even after the + introduction is performed. With this feature the circuit will stay alive for + the same duration as normal web circuits before they expire (usually 10 + minutes). + +3.3.3. Client-side rendezvous circuit hiding + + Following a similar argument as for intro circuits, we are aiming for padded + rendezvous circuits to blend in with the initial cell sequence of general + circuits which usually look like this: + + [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [BEGIN] -> CONNECTED + -> [DATA] -> [DATA] -> DATA -> DATA...(incoming cells continue) + + Whereas normal rendezvous circuits usually look like: + + [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [EST_REND] -> REND_EST + -> REND2 -> [BEGIN] + + This means that up to the sixth cell (the first line), both general and + rend circuits have identical cell sequences. + + After that we want to mimic a [DATA] -> [DATA] -> DATA -> DATA sequence. + + With padding negotiation right after the REND_ESTABLISHED, the sequence + becomes: + + [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [EST_REND] -> REND_EST + -> [PADDING_NEGOTIATE] -> [DROP] -> PADDING_NEGOTIATED -> DROP... + + After which normal application DATA cells continue on the circuit. + + Hence this way we make rendezvous circuits look like general circuits up + till the end of the circuit setup. + + After that our machine gets deactivated, and we let the actual rendezvous + circuit shape the traffic flow. Since rendezvous circuits usually imitate + general circuits (their purpose is to surf the web), we can expect that they + will look alike. + +3.3.4. Circuit setup machine overhead + + For the intro circuit case, we see that the origin-side machine just sends a + single [PADDING_NEGOTIATE] cell, whereas the origin-side machine sends a + PADDING_NEGOTIATED cell and between 7 to 10 DROP cells. This means that the + average overhead of this machine is 11 padding cells per introduction circuit. + + For the rend circuit case, this machine is quite light. Both sides send 2 + padding cells, for a total of 4 padding cells. + +3.4. Circuit padding consensus parameters + + The circuit padding system has a handful of consensus parameters that can + either disable circuit padding entirely, or rate limit the total overhead + at relays and clients. + + * circpad_padding_disabled + - If set to 1, no circuit padding machines will negotiate, and all + current padding machines will cease padding immediately. + - Default: 0 + + * circpad_padding_reduced + - If set to 1, only circuit padding machines marked as "reduced"/"low + overhead" will be used. (Currently no such machines are marked + as "reduced overhead"). + - Default: 0 + + * circpad_global_allowed_cells + - This is the number of padding cells that must be sent before + the 'circpad_global_max_padding_percent' parameter is applied. + - Default: 0 + + * circpad_global_max_padding_percent + - This is the maximum ratio of padding cells to total cells, specified + as a percent. If the global ratio of padding cells to total cells + across all circuits exceeds this percent value, no more padding is sent + until the ratio becomes lower. 0 means no limit. + - Default: 0 + + * circpad_max_circ_queued_cells + - This is the maximum number of cells that can be in the circuitmux queue + before padding stops being sent on that circuit. + - Default: CIRCWINDOW_START_MAX (1000) + + +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 +15. http://arxiv.org/pdf/1512.00524 +16. https://www.cs.kau.se/pulls/hot/thebasketcase-ape/ +17. https://github.com/torproject/tor/tree/master/doc/HACKING/CircuitPaddingDevelopment.md +18. https://www.usenix.org/node/190967 + https://blog.torproject.org/technical-summary-usenix-fingerprinting-paper + |