commit 9f763ae3a3f5271443741a6ed7fe1351708de6af Author: Nick Mathewson <nickm@torproject.org> Date: Wed Oct 14 10:25:31 2020 -0400 Describe our algorithm for waiting between directory retries Closes torspec#25 --- dir-spec.txt | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 61 insertions(+) diff --git a/dir-spec.txt b/dir-spec.txt index dbb15ab..1aca15b 100644 --- a/dir-spec.txt +++ b/dir-spec.txt @@ -3448,6 +3448,10 @@ FPRLIST is +-separated list of recognized authority identity fingerprints as in appendix B. +4.6 Retrying failed downloads + + See section 5.5 below; it applies to caches as well as clients. + 5. Client operation Every Tor that is not a directory server (that is, those that do @@ -3658,6 +3662,63 @@ (This section is removed; authorities no longer assign the 'Named' flag.) +5.5. Retrying failed downloads + + This section applies to caches as well as to clients. + + When a client fails to download a resource (a consensus, a router + descriptor, a microdescriptor, etc) it waits for a certain amount of + time before retrying the download. To determine the amount of time + to wait, clients use a randomized exponential backoff algorithm. + (Specifically, they use a variation of the "decorrelated jitter" + algorithm from + https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/ .) + + The specific formula used to compute the 'i+1'th delay is: + + Delay_{i+1} = MIN(cap, random_between(lower_bound, upper_bound))) + where upper_bound = MAX(lower_bound+1, Delay_i * 3) + lower_bound = MAX(1, base_delay). + + The value of 'cap' is set to INT_MAX; the value of 'base_delay' + depends on what is being downloaded, whether the client is fully + bootstrapped, how the client is configured, and where it is + downloading from. Current base_delay values are: + + Consensus objects, as a non-bridge cache: + 0 (TestingServerConsensusDownloadInitialDelay) + + Consensus objects, as a client or bridge that has bootstrapped: + 0 (TestingClientConsensusDownloadInitialDelay) + + Consensus objects, as a client or bridge that is bootstrapping, + when connecting to an authority because no "fallback" caches are + known: + 0 (ClientBootstrapConsensusAuthorityOnlyDownloadInitialDelay) + + Consensus objects, as a client or bridge that is bootstrapping, + when "fallback" caches are known but connecting to an authority + anyway: + 6 (ClientBootstrapConsensusAuthorityDownloadInitialDelay) + + Consensus objects, as a client or bridge that is bootstrapping, + when downloading from a "fallback" cache. + 0 (ClientBootstrapConsensusFallbackDownloadInitialDelay) + + Bridge descriptors, as a bridge-using client when at least one bridge + is usable: + 10800 (TestingBridgeDownloadInitialDelay) + + Bridge descriptors, otherwise: + 0 (TestingBridgeBootstrapDownloadInitialDelay) + + Other objects, as cache or authority: + 0 (TestingServerDownloadInitialDelay) + + Other objects, as client: + 0 (TestingClientDownloadInitialDelay) + + 6. Standards compliance All clients and servers MUST support HTTP 1.0. Clients and servers MAY