commit 1413093be9d575a4b0805e0328e2867319a543ae Author: Nick Mathewson nickm@torproject.org Date: Mon Jun 27 15:58:45 2011 -0400
Add proposal 182 ("Credit Bucket") by Tschorsch and Scheuermann --- proposals/000-index.txt | 2 + proposals/182-creditbucket.txt | 181 ++++++++++++++++++++++++++++++++++++++++ 2 files changed, 183 insertions(+), 0 deletions(-)
diff --git a/proposals/000-index.txt b/proposals/000-index.txt index 06e97b5..43502ec 100644 --- a/proposals/000-index.txt +++ b/proposals/000-index.txt @@ -102,6 +102,7 @@ Proposals by number: 179 TLS certificate and parameter normalization [DRAFT] 180 Pluggable transports for circumvention [OPEN] 181 Optimistic Data for Tor: Client Side [OPEN] +182 Credit Bucket [DRAFT]
Proposals by status: @@ -114,6 +115,7 @@ Proposals by status: 144 Increase the diversity of circuits by detecting nodes belonging the same provider 175 Automatically promoting Tor clients to nodes 179 TLS certificate and parameter normalization [for 0.2.3.x] + 182 Credit Bucket NEEDS-REVISION: 118 Advertising multiple ORPorts at once 131 Help users to verify they are using Tor diff --git a/proposals/182-creditbucket.txt b/proposals/182-creditbucket.txt new file mode 100644 index 0000000..8796bd1 --- /dev/null +++ b/proposals/182-creditbucket.txt @@ -0,0 +1,181 @@ +Filename: 182-creditbucket.txt +Title: Credit Bucket +Author: Florian Tschorsch and Björn Scheuermann +Created: 22 Jun 2011 +Status: Draft + +Overview: + + The following proposal targets the reduction of queuing times in onion + routers. In particular, we focus on the token bucket algorithm in Tor and + point out that current usage unnecessarily locks cells for long time spans. + We propose a non-intrusive change in Tor's design which overcomes the + deficiencies. + +Motivation and Background: + + Cell statistics from the Tor network [1] reveal that cells reside in + individual onion routers' cell queues for up to several seconds. These + queuing times increase the end-to-end delay very significantly and are + apparently the largest contributor to overall cell latency in Tor. + + In Tor there exist multiple token buckets on different logical levels. They + all work independently. They are used to limit the up- and downstream of an + onion router. All token buckets are refilled every second with a constant + amount of tokens that depends on the configured bandwidth limits. For + example, the so-called RelayedTokenBucket limits relay traffic only. All + read data of incoming connections are bound to a dedicated read token + bucket. An analogous mechanism exists for written data leaving the onion + router. We were able to identify the specific usage and implementation of + the token bucket algorithm as one cause for very high (and unnecessary) + queuing times in an onion router. + + We observe that the token buckets in Tor are (surprisingly at a first + glance) allowed to take on negative fill levels. This is justified by the + TLS connections between onion routers where whole TLS records need to be + processed. The token bucket on the incoming side (i.e., the one which + determines at which rate it is allowed to read from incoming TCP + connections) in particular often runs into non-negligible negative fill + levels. As a consequence of this behavior, sometimes slightly more data is + read than it would be admissible upon strict interpretation of the token + bucket concept. + + However, the token bucket for limiting the outgoing rate does not take on + negative fill levels equally often. Consequently, it regularly happens + that somewhat more data are read on the incoming side than the outgoing + token bucket allows to be written during the same cycle, even if their + configured data rates are the same. The respective cells will thus not be + allowed to leave the onion router immediately. They will thus necessarily + be queued for at least as long as it takes until the token bucket on the + outgoing side is refilled again. The refill interval currently is, as + mentioned before, one second -- so, these cells are delayed for a very + substantial time. In summary, one could say that the two buckets, on the + incoming and outgoing side, work like a double door system and frequently + lock cells for a full token bucket refill interval length. + +General Design: + + In order to overcome the described problem, we propose the following + changes related to the token bucket algorithm. + + We observe that the token bucket on the outgoing connections with its + current design is contra productive in the sense of queuing times. We + therefore propose modifications to the token bucket algorithm that will + eliminate the "double door effect" discussed above. + + Let us start from Tor's current approach: Thus, we have a regular token + bucket on the reading side with a certain rate and a certain burst size. + Let x denote the current amount of tokens in the bucket. On the outgoing + side we need something appropriate that monitors and constrains the + outgoing rate, but at the same time avoids holding back cells (cf. double + door effects) whenever possible. + + Here we propose something that adopts the role of a token bucket, but + realizes this functionality in a slightly different way. We call it a + "credit bucket". Like a token bucket, the credit bucket also has a current + fill level, denoted by y. However, the credit bucket is refilled in a + different way. + + To understand how it works, let us look at the possible operations: + + As said, x is the fill level of a regular token bucket on the incoming + side and thus gets incremented periodically according to the configured + rate. No changes here. + + If x<=0, we are obviously not allowed to read. If x>0, we are allowed to + read up to x bytes of incoming data. If k bytes are read (k<=x), then we + update x and y as follows: + + x = x - k (1) + y = y + k (2) + + (1) is the standard token bucket operation on the incoming side. Whenever + data is admitted in, though, an additional operation is performed: (2) + allocates the same number of bytes on the outgoing side, which will later + on allow the same number of bytes to leave the onion router without any + delays. + + If y + x > -M, we are allowed to write up to y + x + M bytes on the + outgoing side, where M is a positive constant. M specifies a burst size for + the outgoing side. M should be higher than the number of tokens that get + refilled during a refill interval, we would suggest to have M in the order + of a few seconds "worth" of data. Now if k bytes are written on the + outgoing side, we proceed as follows: + + If k <= y then y = y - k + + In this case we use "saved" credits, previously allocated on the incoming + side when incoming data has been processed. + + If k > y then y = 0 and x = x - (k-y) + + We generated additional traffic in the onion router, so that more data is + to be sent than has been read (the credit is not sufficient). We therefore + "steal" tokens from the token buffer on the incoming side to compensate for + the additionally generated data. This will result in correspondingly less + data being read on the incoming side subsequently. As a result of such an + operation, the token bucket fill level x on the incoming side may become + negative (but it can never fall below -M). + + If y + x <= -M then outgoing data will be held back. This may lead to + double-door effects, but only in extreme cases where the outgoing traffic + largely exceeds the incoming traffic, so that the outgoing bursts size M is + exceeded. + + Aside from short-term bursts of configurable size (as with every token + bucket), this procedure guarantees that the configured rate may never be + exceeded (on the application layer, that is; as with the current + implementation, an attacker may easily cause the onion router to + arbitrarily exceed the limits on the lower layers). Over time, we never + send more data than the configured rate: every sent byte needs a + corresponding token on the incoming side; this token must either have been + consumed by an incoming byte before (it then became a "credit"), or it is + "stolen" from the incoming bucket to compensate for data generated within + the onion router. + +Specific Design Changes: + + In the following we briefly point out the specific changes that need to be + done in Tor's source code. By doing so one can see how non intrusive our + modifications are. + + First we need to address the bucket increment and decrement operations. + According to the described logic above, this should be done in the methods + connection_bucket_refill and connection_buckets_decrement respectively. In + particular allocating, saving and "stealing" of tokens need to be + considered here. + + Second the rate limiting, i.e. the amount we are allowed to write + (connection_bucket_write_limit) needs to be adapted in lines of the credit + bucket logic. Meaning in order to avoid the here identified unnecessary + queuing of cells, we need to consider the new burst parameter M. Here we + also need to take non rate limited connections such as from the localhost + into account. The rate limiting on the reading side remains the same. + + At last we need to find good values/ ratios for the parameter M such that + the trade off between avoiding "double door effects" and maintaining + strict rate limits work as expected. As future work and after insights + about the performance gain of the here described proposal we need to find a + way to implement this both using bufferevent rate limiting with libevent + 2.3.x and Tor's rate limiting code. + +Conclusion: + + This proposal can be implemented with moderate effort and requires changes + only at the points where currently the token bucket operations are + performed. + + We feel that this is not the be-all and end-all solution, because it again + introduces a feedback loop between the incoming and the outgoing side. We + therefore still hope that we will be able to come to a both simpler and + more effective design in the future. However, we believe that what we + proposed here is a good compromise between avoiding double-door effects to + the furthest possible extent, strictly enforcing an application-layer data + rate, and keeping the extent of changes to the code small. + + Feedback is highly appreciated. + +References: + + [1] Karsten Loesing. Analysis of Circuit Queues in Tor. August 25, 2009. + [2] https://trac.torproject.org/projects/tor/wiki/sponsors/SponsorD/June2011