Merge remote-tracking branch 'tor-gitlab/mr/12' into master

   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

   (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