diff --git a/framework/ratelimiter.py b/framework/ratelimiter.py
new file mode 100644
index 0000000..b2bbb25
--- /dev/null
+++ b/framework/ratelimiter.py
@@ -0,0 +1,292 @@
+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file or at
+# https://developers.google.com/open-source/licenses/bsd
+
+"""Request rate limiting implementation.
+
+This is intented to be used for automatic DDoS protection.
+
+"""
+from __future__ import print_function
+from __future__ import division
+from __future__ import absolute_import
+
+import datetime
+import logging
+import os
+import settings
+import time
+
+from infra_libs import ts_mon
+
+from google.appengine.api import memcache
+from google.appengine.api.modules import modules
+from google.appengine.api import users
+
+from services import client_config_svc
+
+
+N_MINUTES = 5
+EXPIRE_AFTER_SECS = 60 * 60
+DEFAULT_LIMIT = 60 * N_MINUTES  # 300 page requests in 5 minutes is 1 QPS.
+DEFAULT_API_QPM = 1000  # For example, chromiumdash uses ~64 per page, 8s each.
+
+ANON_USER = 'anon'
+
+COUNTRY_HEADER = 'X-AppEngine-Country'
+
+COUNTRY_LIMITS = {
+  # Two-letter country code: max requests per N_MINUTES
+  # This limit will apply to all requests coming
+  # from this country.
+  # To add a country code, see GAE logs and use the
+  # appropriate code from https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
+  # E.g., 'cn': 300,  # Limit to 1 QPS.
+}
+
+# Modules not in this list will not have rate limiting applied by this
+# class.
+MODULE_ALLOWLIST = ['default', 'api']
+
+
+def _CacheKeys(request, now_sec):
+  """ Returns an array of arrays. Each array contains strings with
+      the same prefix and a timestamp suffix, starting with the most
+      recent and decrementing by 1 minute each time.
+  """
+  now = datetime.datetime.fromtimestamp(now_sec)
+  country = request.headers.get(COUNTRY_HEADER, 'ZZ')
+  ip = request.remote_addr
+  minute_buckets = [now - datetime.timedelta(minutes=m) for m in
+                    range(N_MINUTES)]
+  user = users.get_current_user()
+  user_email = user.email() if user else ANON_USER
+
+  # <IP, country, user_email> to be rendered into each key prefix.
+  prefixes = []
+
+  # All logged-in users get a per-user rate limit, regardless of IP and country.
+  if user:
+    prefixes.append(['ALL', 'ALL', user.email()])
+  else:
+    # All anon requests get a per-IP ratelimit.
+    prefixes.append([ip, 'ALL', 'ALL'])
+
+  # All requests from a problematic country get a per-country rate limit,
+  # regardless of the user (even a non-logged-in one) or IP.
+  if country in COUNTRY_LIMITS:
+    prefixes.append(['ALL', country, 'ALL'])
+
+  keysets = []
+  for prefix in prefixes:
+    keysets.append(['ratelimit-%s-%s' % ('-'.join(prefix),
+        str(minute_bucket.replace(second=0, microsecond=0)))
+        for minute_bucket in minute_buckets])
+
+  return keysets, country, ip, user_email
+
+
+def _CreateApiCacheKeys(client_id, client_email, now_sec):
+  country = os.environ.get('HTTP_X_APPENGINE_COUNTRY')
+  ip = os.environ.get('REMOTE_ADDR')
+  now = datetime.datetime.fromtimestamp(now_sec)
+  minute_buckets = [now - datetime.timedelta(minutes=m) for m in
+                    range(N_MINUTES)]
+  minute_strs = [str(minute_bucket.replace(second=0, microsecond=0))
+                 for minute_bucket in minute_buckets]
+  keys = []
+
+  if client_id and client_id != 'anonymous':
+    keys.append(['apiratelimit-%s-%s' % (client_id, minute_str)
+                 for minute_str in minute_strs])
+  if client_email:
+    keys.append(['apiratelimit-%s-%s' % (client_email, minute_str)
+                 for minute_str in minute_strs])
+  else:
+    keys.append(['apiratelimit-%s-%s' % (ip, minute_str)
+                 for minute_str in minute_strs])
+    if country in COUNTRY_LIMITS:
+      keys.append(['apiratelimit-%s-%s' % (country, minute_str)
+                   for minute_str in minute_strs])
+
+  return keys
+
+
+class RateLimiter(object):
+
+  blocked_requests = ts_mon.CounterMetric(
+      'monorail/ratelimiter/blocked_request',
+      'Count of requests that exceeded the rate limit and were blocked.',
+      None)
+  limit_exceeded = ts_mon.CounterMetric(
+      'monorail/ratelimiter/rate_exceeded',
+      'Count of requests that exceeded the rate limit.',
+      None)
+  cost_thresh_exceeded = ts_mon.CounterMetric(
+      'monorail/ratelimiter/cost_thresh_exceeded',
+      'Count of requests that were expensive to process',
+      None)
+  checks = ts_mon.CounterMetric(
+      'monorail/ratelimiter/check',
+      'Count of checks done, by fail/success type.',
+      [ts_mon.StringField('type')])
+
+  def __init__(self, _cache=memcache, fail_open=True, **_kwargs):
+    self.fail_open = fail_open
+
+  def CheckStart(self, request, now=None):
+    if (modules.get_current_module_name() not in MODULE_ALLOWLIST or
+        users.is_current_user_admin()):
+      return
+    logging.info('X-AppEngine-Country: %s' %
+      request.headers.get(COUNTRY_HEADER, 'ZZ'))
+
+    if now is None:
+      now = time.time()
+
+    keysets, country, ip, user_email  = _CacheKeys(request, now)
+    # There are either two or three sets of keys in keysets.
+    # Three if the user's country is in COUNTRY_LIMITS, otherwise two.
+    self._AuxCheckStart(
+        keysets, COUNTRY_LIMITS.get(country, DEFAULT_LIMIT),
+        settings.ratelimiting_enabled,
+        RateLimitExceeded(country=country, ip=ip, user_email=user_email))
+
+  def _AuxCheckStart(self, keysets, limit, ratelimiting_enabled,
+                     exception_obj):
+    for keys in keysets:
+      count = 0
+      try:
+        counters = memcache.get_multi(keys)
+        count = sum(counters.values())
+        self.checks.increment({'type': 'success'})
+      except Exception as e:
+        logging.error(e)
+        if not self.fail_open:
+          self.checks.increment({'type': 'fail_closed'})
+          raise exception_obj
+        self.checks.increment({'type': 'fail_open'})
+
+      if count > limit:
+        # Since webapp2 won't let us return a 429 error code
+        # <http://tools.ietf.org/html/rfc6585#section-4>, we can't
+        # monitor rate limit exceeded events with our standard tools.
+        # We return a 400 with a custom error message to the client,
+        # and this logging is so we can monitor it internally.
+        logging.info('%s, %d' % (exception_obj.message, count))
+
+        self.limit_exceeded.increment()
+
+        if ratelimiting_enabled:
+          self.blocked_requests.increment()
+          raise exception_obj
+
+      k = keys[0]
+      # Only update the latest *time* bucket for each prefix (reverse chron).
+      memcache.add(k, 0, time=EXPIRE_AFTER_SECS)
+      memcache.incr(k, initial_value=0)
+
+  def CheckEnd(self, request, now, start_time):
+    """If a request was expensive to process, charge some extra points
+    against this set of buckets.
+    We pass in both now and start_time so we can update the buckets
+    based on keys created from start_time instead of now.
+    now and start_time are float seconds.
+    """
+    if (modules.get_current_module_name() not in MODULE_ALLOWLIST):
+      return
+
+    elapsed_ms = int((now - start_time) * 1000)
+    # Would it kill the python lib maintainers to have timedelta.total_ms()?
+    penalty = elapsed_ms // settings.ratelimiting_ms_per_count - 1
+    if penalty >= 1:
+      # TODO: Look into caching the keys instead of generating them twice
+      # for every request. Say, return them from CheckStart so they can
+      # be passed back in here later.
+      keysets, country, ip, user_email = _CacheKeys(request, start_time)
+
+      self._AuxCheckEnd(
+          keysets,
+          'Rate Limit Cost Threshold Exceeded: %s, %s, %s' % (
+              country, ip, user_email),
+          penalty)
+
+  def _AuxCheckEnd(self, keysets, log_str, penalty):
+    self.cost_thresh_exceeded.increment()
+    for keys in keysets:
+      logging.info(log_str)
+
+      # Only update the latest *time* bucket for each prefix (reverse chron).
+      k = keys[0]
+      memcache.add(k, 0, time=EXPIRE_AFTER_SECS)
+      memcache.incr(k, delta=penalty, initial_value=0)
+
+
+class ApiRateLimiter(RateLimiter):
+
+  blocked_requests = ts_mon.CounterMetric(
+      'monorail/apiratelimiter/blocked_request',
+      'Count of requests that exceeded the rate limit and were blocked.',
+      None)
+  limit_exceeded = ts_mon.CounterMetric(
+      'monorail/apiratelimiter/rate_exceeded',
+      'Count of requests that exceeded the rate limit.',
+      None)
+  cost_thresh_exceeded = ts_mon.CounterMetric(
+      'monorail/apiratelimiter/cost_thresh_exceeded',
+      'Count of requests that were expensive to process',
+      None)
+  checks = ts_mon.CounterMetric(
+      'monorail/apiratelimiter/check',
+      'Count of checks done, by fail/success type.',
+      [ts_mon.StringField('type')])
+
+  #pylint: disable=arguments-differ
+  def CheckStart(self, client_id, client_email, now=None):
+    if now is None:
+      now = time.time()
+
+    keysets = _CreateApiCacheKeys(client_id, client_email, now)
+    qpm_limit = client_config_svc.GetQPMDict().get(
+        client_email, DEFAULT_API_QPM)
+    if qpm_limit < DEFAULT_API_QPM:
+      qpm_limit = DEFAULT_API_QPM
+    window_limit = qpm_limit * N_MINUTES
+    self._AuxCheckStart(
+        keysets, window_limit,
+        settings.api_ratelimiting_enabled,
+        ApiRateLimitExceeded(client_id, client_email))
+
+  #pylint: disable=arguments-differ
+  def CheckEnd(self, client_id, client_email, now, start_time):
+
+    elapsed_ms = int((now - start_time) * 1000)
+    penalty = elapsed_ms // settings.ratelimiting_ms_per_count - 1
+
+    if penalty >= 1:
+      keysets = _CreateApiCacheKeys(client_id, client_email, start_time)
+      self._AuxCheckEnd(
+          keysets,
+          'API Rate Limit Cost Threshold Exceeded: %s, %s' % (
+              client_id, client_email),
+          penalty)
+
+
+class RateLimitExceeded(Exception):
+  def __init__(self, country=None, ip=None, user_email=None, **_kwargs):
+    self.country = country
+    self.ip = ip
+    self.user_email = user_email
+    message = 'RateLimitExceeded: %s, %s, %s' % (
+        self.country, self.ip, self.user_email)
+    super(RateLimitExceeded, self).__init__(message)
+
+
+class ApiRateLimitExceeded(Exception):
+  def __init__(self, client_id, client_email):
+    self.client_id = client_id
+    self.client_email = client_email
+    message = 'RateLimitExceeded: %s, %s' % (
+        self.client_id, self.client_email)
+    super(ApiRateLimitExceeded, self).__init__(message)