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Accurately account for slow server start grace period.

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dachshund 2013-09-09 13:50:11 -04:00
parent 9a3820bfcf
commit 772905a67e
1 changed files with 25 additions and 20 deletions
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45
tuf/download.py
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@ -29,7 +29,7 @@
import logging
import os.path
import socket
import time
import timeit
import tuf
import tuf.conf
@ -77,8 +77,11 @@ def __init__(self, sock, mode='rb', bufsize=-1, close=False):
# Count the number of bytes received with this socket.
self.__number_of_bytes_received = 0
# Count the seconds spent receiving with this socket.
self.__seconds_spent_receiving = 0
# Count the seconds spent receiving with this socket. Tolerate servers with
# a slow start by ignoring their delivery speed for
# tuf.conf.SLOW_START_GRACE_PERIOD seconds.
assert tuf.conf.SLOW_START_GRACE_PERIOD > 0
self.__seconds_spent_receiving = -tuf.conf.SLOW_START_GRACE_PERIOD
# Remember the time a clock was started.
self.__start_time = None
@ -108,8 +111,8 @@ def __start_clock(self):
# We must have reset the clock before this.
assert self.__start_time is None
# We are using wall time, so it will be imprecise sometimes.
self.__start_time = time.time()
# We use (platform-specific) wall time, so it will be imprecise sometimes.
self.__start_time = timeit.default_timer()
@ -139,8 +142,8 @@ def __stop_clock_and_check_speed(self, data_length):
"""
# We are using wall time, so it will be imprecise sometimes.
stop_time = time.time()
# We use (platform-specific) wall time, so it will be imprecise sometimes.
stop_time = timeit.default_timer()
# We must have already started the clock.
assert self.__start_time > 0
time_delta = stop_time-self.__start_time
@ -150,22 +153,24 @@ def __stop_clock_and_check_speed(self, data_length):
# Measure the average download speed.
self.__number_of_bytes_received += data_length
self.__seconds_spent_receiving += time_delta
average_download_speed = \
self.__number_of_bytes_received/self.__seconds_spent_receiving
# If the average download speed is below a certain threshold, we flag this
# as a possible slow-retrieval attack. This threshold will determine our
# bias: if it is too low, we will have more false positives; if it is too
# high, we will have more false negatives.
if average_download_speed < tuf.conf.MIN_AVERAGE_DOWNLOAD_SPEED:
if self.__seconds_spent_receiving <= tuf.conf.SLOW_START_GRACE_PERIOD:
logger.debug('Slow average download speed: '+\
str(average_download_speed)+' bytes/second')
if self.__seconds_spent_receiving > 0:
average_download_speed = \
self.__number_of_bytes_received/self.__seconds_spent_receiving
# If the average download speed is below a certain threshold, we flag this
# as a possible slow-retrieval attack. This threshold will determine our
# bias: if it is too low, we will have more false positives; if it is too
# high, we will have more false negatives.
if average_download_speed < tuf.conf.MIN_AVERAGE_DOWNLOAD_SPEED:
raise tuf.SlowRetrievalError(average_download_speed)
else:
raise tuf.SlowRetrievalError(average_download_speed)
logger.debug('Good average download speed: '+\
str(average_download_speed)+' bytes/second')
else:
logger.debug('Good average download speed: '+\
str(average_download_speed)+' bytes/second')
logger.debug('Ignoring average download speed for another: '+\
str(-self.__seconds_spent_receiving)+' seconds')