[tor-bugs] #25598 [Obfuscation/Snowflake]: Let the broker inform proxies how often to poll

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#25598: Let the broker inform proxies how often to poll
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     Reporter:  dcf                    |      Owner:  (none)
         Type:  enhancement            |     Status:  new
     Priority:  Medium                 |  Milestone:
    Component:  Obfuscation/Snowflake  |    Version:
     Severity:  Normal                 |   Keywords:
Actual Points:                         |  Parent ID:
       Points:                         |   Reviewer:
      Sponsor:                         |
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 Currently, proxies poll the broker at a static rate of once every 5–10
 seconds. If we're anticipating thousands of proxies, we don't need them to
 poll so frequently.

 The broker could instead tell each proxy how long to wait before polling
 again. The broker could even dynamically adjust the rate based on an
 estimate of supply and demand.

 One way to do this would be a custom header in responses to `/proxy`
 requests:
 {{{
 Snowflake-Next-Poll: Thu, 22 Mar 2018 18:05:47 GMT
 }}}
 Or using a relative time offset:
 {{{
 Snowflake-Next-Poll: 600
 }}}

 There was a similar idea for flash proxy.
  #8171::
    The facilitator included a fixed `check-back-in=600` in its responses.
  #8172::
    Adjust polling interval dynamically (never implemented).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25598>
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