ipsec: Performance improvement of ipsec4_output_node using flow cache

Adding flow cache support to improve outbound IPv4/IPSec SPD lookup
performance. Details about flow cache:
  Mechanism:
  1. First packet of a flow will undergo linear search in SPD
     table. Once a policy match is found, a new entry will be added
     into the flow cache. From 2nd packet onwards, the policy lookup
     will happen in flow cache.
  2. The flow cache is implemented using bihash without collision
     handling. This will avoid the logic to age out or recycle the old
     flows in flow cache. Whenever a collision occurs, old entry will
     be overwritten by the new entry. Worst case is when all the 256
     packets in a batch result in collision and fall back to linear
     search. Average and best case will be O(1).
  3. The size of flow cache is fixed and decided based on the number
     of flows to be supported. The default is set to 1 million flows.
     This can be made as a configurable option as a next step.
  4. Whenever a SPD rule is added/deleted by the control plane, the
     flow cache entries will be completely deleted (reset) in the
     control plane. The assumption here is that SPD rule add/del is not
     a frequent operation from control plane. Flow cache reset is done,
     by putting the data plane in fall back mode, to bypass flow cache
     and do linear search till the SPD rule add/delete operation is
     complete. Once the rule is successfully added/deleted, the data
     plane will be allowed to make use of the flow cache. The flow
     cache will be reset only after flushing out the inflight packets
     from all the worker cores using
     vlib_worker_wait_one_loop().

  Details about bihash usage:
  1. A new bihash template (16_8) is added to support IPv4 5 tuple.
     BIHASH_KVP_PER_PAGE and BIHASH_KVP_AT_BUCKET_LEVEL are set
     to 1 in the new template. It means only one KVP is supported
     per bucket.
  2. Collision handling is avoided by calling
     BV (clib_bihash_add_or_overwrite_stale) function.
     Through the stale callback function pointer, the KVP entry
     will be overwritten during collision.
  3. Flow cache reset is done using
     BV (clib_bihash_foreach_key_value_pair) function.
     Through the callback function pointer, the KVP value is reset
     to ~0ULL.

  MRR performance numbers with 1 core, 1 ESP Tunnel, null-encrypt,
  64B for different SPD policy matching indices:

  SPD Policy index    : 1          10         100        1000
  Throughput          : MPPS/MPPS  MPPS/MPPS  MPPS/MPPS  KPPS/MPPS
  (Baseline/Optimized)
  ARM Neoverse N1     : 5.2/4.84   4.55/4.84  2.11/4.84  329.5/4.84
  ARM TX2             : 2.81/2.6   2.51/2.6   1.27/2.6   176.62/2.6
  INTEL SKX           : 4.93/4.48  4.29/4.46  2.05/4.48  336.79/4.47

  Next Steps:
  Following can be made as a configurable option through startup
  conf at IPSec level:
  1. Enable/Disable Flow cache.
  2. Bihash configuration like number of buckets and memory size.
  3. Dual/Quad loop unroll can be applied around bihash to further
     improve the performance.
  4. The same flow cache logic can be applied for IPv6 as well as in
     IPSec inbound direction. A deeper and wider flow cache using
     bihash_40_8 can replace existing bihash_16_8, to make it
     common for both IPv4 and IPv6 in both outbound and
     inbound directions.

Following changes are made based on the review comments:
1. ON/OFF flow cache through startup conf. Default: OFF
2. Flow cache stale entry detection using epoch counter.
3. Avoid host order endianness conversion during flow cache
   lookup.
4. Move IPSec startup conf to a common file.
5. Added SPD flow cache unit test case
6. Replaced bihash with vectors to implement flow cache.
7. ipsec_add_del_policy API is not mpsafe. Cleaned up
   inflight packets check in control plane.

Type: improvement
Signed-off-by: mgovind <govindarajan.Mohandoss@arm.com>
Signed-off-by: Zachary Leaf <zachary.leaf@arm.com>
Tested-by: Jieqiang Wang <jieqiang.wang@arm.com>
Change-Id: I62b4d6625fbc6caf292427a5d2046aa5672b2006

1 files changed, 583 insertions, 0 deletions

diff --git a/test/test_ipsec_spd_flow_cache.py b/test/test_ipsec_spd_flow_cache.py
new file mode 100644
index 00000000000..0c26e7b9e6a
--- /dev/null
+++ b/test/test_ipsec_spd_flow_cache.py
@@ -0,0 +1,583 @@
+import socket
+import unittest
+
+from util import ppp
+from framework import VppTestRunner
+from template_ipsec import SpdFlowCacheTemplate
+
+
+class SpdFlowCacheOutbound(SpdFlowCacheTemplate):
+    # Override setUpConstants to enable outbound flow cache in config
+    @classmethod
+    def setUpConstants(cls):
+        super(SpdFlowCacheOutbound, cls).setUpConstants()
+        cls.vpp_cmdline.extend(["ipsec", "{",
+                                "ipv4-outbound-spd-flow-cache on",
+                                "}"])
+        cls.logger.info("VPP modified cmdline is %s" % " "
+                        .join(cls.vpp_cmdline))
+
+
+class IPSec4SpdTestCaseAdd(SpdFlowCacheOutbound):
+    """ IPSec/IPv4 outbound: Policy mode test case with flow cache \
+        (add rule)"""
+    def test_ipsec_spd_outbound_add(self):
+        # In this test case, packets in IPv4 FWD path are configured
+        # to go through IPSec outbound SPD policy lookup.
+        # 2 SPD rules (1 HIGH and 1 LOW) are added.
+        # High priority rule action is set to BYPASS.
+        # Low priority rule action is set to DISCARD.
+        # Traffic sent on pg0 interface should match high priority
+        # rule and should be sent out on pg1 interface.
+        self.create_interfaces(2)
+        pkt_count = 5
+        self.spd_create_and_intf_add(1, [self.pg1])
+        policy_0 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        policy_1 = self.spd_add_rem_policy(  # outbound, priority 5
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=5, policy_type="discard")
+
+        # check flow cache is empty before sending traffic
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # create the packet stream
+        packets = self.create_stream(self.pg0, self.pg1, pkt_count)
+        # add the stream to the source interface + enable capture
+        self.pg0.add_stream(packets)
+        self.pg0.enable_capture()
+        self.pg1.enable_capture()
+        # start the packet generator
+        self.pg_start()
+        # get capture
+        capture = self.pg1.get_capture()
+        for packet in capture:
+            try:
+                self.logger.debug(ppp("SPD - Got packet:", packet))
+            except Exception:
+                self.logger.error(ppp("Unexpected or invalid packet:", packet))
+                raise
+        self.logger.debug("SPD: Num packets: %s", len(capture.res))
+
+        # assert nothing captured on pg0
+        self.pg0.assert_nothing_captured()
+        # verify captured packets
+        self.verify_capture(self.pg0, self.pg1, capture)
+        # verify all policies matched the expected number of times
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(0, policy_1)
+        # check policy in SPD has been cached after traffic
+        # matched BYPASS rule in SPD
+        self.verify_num_outbound_flow_cache_entries(1)
+
+
+class IPSec4SpdTestCaseRemove(SpdFlowCacheOutbound):
+    """ IPSec/IPv4 outbound: Policy mode test case with flow cache \
+        (remove rule)"""
+    def test_ipsec_spd_outbound_remove(self):
+        # In this test case, packets in IPv4 FWD path are configured
+        # to go through IPSec outbound SPD policy lookup.
+        # 2 SPD rules (1 HIGH and 1 LOW) are added.
+        # High priority rule action is set to BYPASS.
+        # Low priority rule action is set to DISCARD.
+        # High priority rule is then removed.
+        # Traffic sent on pg0 interface should match low priority
+        # rule and should be discarded after SPD lookup.
+        self.create_interfaces(2)
+        pkt_count = 5
+        self.spd_create_and_intf_add(1, [self.pg1])
+        policy_0 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        policy_1 = self.spd_add_rem_policy(  # outbound, priority 5
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=5, policy_type="discard")
+
+        # check flow cache is empty before sending traffic
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # create the packet stream
+        packets = self.create_stream(self.pg0, self.pg1, pkt_count)
+        # add the stream to the source interface + enable capture
+        self.pg0.add_stream(packets)
+        self.pg0.enable_capture()
+        self.pg1.enable_capture()
+        # start the packet generator
+        self.pg_start()
+        # get capture
+        capture = self.pg1.get_capture()
+        for packet in capture:
+            try:
+                self.logger.debug(ppp("SPD - Got packet:", packet))
+            except Exception:
+                self.logger.error(ppp("Unexpected or invalid packet:", packet))
+                raise
+
+        # assert nothing captured on pg0
+        self.pg0.assert_nothing_captured()
+        # verify capture on pg1
+        self.logger.debug("SPD: Num packets: %s", len(capture.res))
+        self.verify_capture(self.pg0, self.pg1, capture)
+        # verify all policies matched the expected number of times
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(0, policy_1)
+        # check policy in SPD has been cached after traffic
+        # matched BYPASS rule in SPD
+        self.verify_num_outbound_flow_cache_entries(1)
+
+        # now remove the bypass rule
+        self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass",
+            remove=True)
+        # verify flow cache counter has been reset by rule removal
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # resend the same packets
+        self.pg0.add_stream(packets)
+        self.pg0.enable_capture()  # flush the old captures
+        self.pg1.enable_capture()
+        self.pg_start()
+        # assert nothing captured on pg0
+        self.pg0.assert_nothing_captured()
+        # all packets will be dropped by SPD rule
+        self.pg1.assert_nothing_captured()
+        # verify all policies matched the expected number of times
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(pkt_count, policy_1)
+        # previous stale entry in flow cache should have been overwritten,
+        # with one active entry
+        self.verify_num_outbound_flow_cache_entries(1)
+
+
+class IPSec4SpdTestCaseReadd(SpdFlowCacheOutbound):
+    """ IPSec/IPv4 outbound: Policy mode test case with flow cache \
+        (add, remove, re-add)"""
+    def test_ipsec_spd_outbound_readd(self):
+        # In this test case, packets in IPv4 FWD path are configured
+        # to go through IPSec outbound SPD policy lookup.
+        # 2 SPD rules (1 HIGH and 1 LOW) are added.
+        # High priority rule action is set to BYPASS.
+        # Low priority rule action is set to DISCARD.
+        # Traffic sent on pg0 interface should match high priority
+        # rule and should be sent out on pg1 interface.
+        # High priority rule is then removed.
+        # Traffic sent on pg0 interface should match low priority
+        # rule and should be discarded after SPD lookup.
+        # Readd high priority rule.
+        # Traffic sent on pg0 interface should match high priority
+        # rule and should be sent out on pg1 interface.
+        self.create_interfaces(2)
+        pkt_count = 5
+        self.spd_create_and_intf_add(1, [self.pg1])
+        policy_0 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        policy_1 = self.spd_add_rem_policy(  # outbound, priority 5
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=5, policy_type="discard")
+
+        # check flow cache is empty before sending traffic
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # create the packet stream
+        packets = self.create_stream(self.pg0, self.pg1, pkt_count)
+        # add the stream to the source interface + enable capture
+        self.pg0.add_stream(packets)
+        self.pg0.enable_capture()
+        self.pg1.enable_capture()
+        # start the packet generator
+        self.pg_start()
+        # get capture
+        capture = self.pg1.get_capture()
+        for packet in capture:
+            try:
+                self.logger.debug(ppp("SPD - Got packet:", packet))
+            except Exception:
+                self.logger.error(ppp("Unexpected or invalid packet:", packet))
+                raise
+        self.logger.debug("SPD: Num packets: %s", len(capture.res))
+
+        # assert nothing captured on pg0
+        self.pg0.assert_nothing_captured()
+        # verify capture on pg1
+        self.verify_capture(self.pg0, self.pg1, capture)
+        # verify all policies matched the expected number of times
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(0, policy_1)
+        # check policy in SPD has been cached after traffic
+        # matched BYPASS rule in SPD
+        self.verify_num_outbound_flow_cache_entries(1)
+
+        # now remove the bypass rule, leaving only the discard rule
+        self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass",
+            remove=True)
+        # verify flow cache counter has been reset by rule removal
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # resend the same packets
+        self.pg0.add_stream(packets)
+        self.pg0.enable_capture()  # flush the old captures
+        self.pg1.enable_capture()
+        self.pg_start()
+
+        # assert nothing captured on pg0
+        self.pg0.assert_nothing_captured()
+        # all packets will be dropped by SPD rule
+        self.pg1.assert_nothing_captured()
+        # verify all policies matched the expected number of times
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(pkt_count, policy_1)
+        # previous stale entry in flow cache should have been overwritten
+        self.verify_num_outbound_flow_cache_entries(1)
+
+        # now readd the bypass rule
+        policy_0 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        # verify flow cache counter has been reset by rule addition
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # resend the same packets
+        self.pg0.add_stream(packets)
+        self.pg0.enable_capture()  # flush the old captures
+        self.pg1.enable_capture()
+        self.pg_start()
+
+        # get capture
+        capture = self.pg1.get_capture(pkt_count)
+        for packet in capture:
+            try:
+                self.logger.debug(ppp("SPD - Got packet:", packet))
+            except Exception:
+                self.logger.error(ppp("Unexpected or invalid packet:", packet))
+                raise
+        self.logger.debug("SPD: Num packets: %s", len(capture.res))
+
+        # assert nothing captured on pg0
+        self.pg0.assert_nothing_captured()
+        # verify captured packets
+        self.verify_capture(self.pg0, self.pg1, capture)
+        # verify all policies matched the expected number of times
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(pkt_count, policy_1)
+        # previous stale entry in flow cache should have been overwritten
+        self.verify_num_outbound_flow_cache_entries(1)
+
+
+class IPSec4SpdTestCaseMultiple(SpdFlowCacheOutbound):
+    """ IPSec/IPv4 outbound: Policy mode test case with flow cache \
+        (multiple interfaces, multiple rules)"""
+    def test_ipsec_spd_outbound_multiple(self):
+        # In this test case, packets in IPv4 FWD path are configured to go
+        # through IPSec outbound SPD policy lookup.
+        # Multiples rules on multiple interfaces are tested at the same time.
+        # 3x interfaces are configured, binding the same SPD to each.
+        # Each interface has 2 SPD rules (1 BYPASS and 1 DISCARD).
+        # On pg0 & pg1, the BYPASS rule is HIGH priority
+        # On pg2, the DISCARD rule is HIGH priority
+        # Traffic should be received on pg0 & pg1 and dropped on pg2.
+        self.create_interfaces(3)
+        pkt_count = 5
+        # bind SPD to all interfaces
+        self.spd_create_and_intf_add(1, self.pg_interfaces)
+        # add rules on all interfaces
+        policy_01 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        policy_02 = self.spd_add_rem_policy(  # outbound, priority 5
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=5, policy_type="discard")
+
+        policy_11 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg1, self.pg2, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        policy_12 = self.spd_add_rem_policy(  # outbound, priority 5
+            1, self.pg1, self.pg2, socket.IPPROTO_UDP,
+            is_out=1, priority=5, policy_type="discard")
+
+        policy_21 = self.spd_add_rem_policy(  # outbound, priority 5
+            1, self.pg2, self.pg0, socket.IPPROTO_UDP,
+            is_out=1, priority=5, policy_type="bypass")
+        policy_22 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg2, self.pg0, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="discard")
+
+        # check flow cache is empty (0 active elements) before sending traffic
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # create the packet streams
+        packets0 = self.create_stream(self.pg0, self.pg1, pkt_count)
+        packets1 = self.create_stream(self.pg1, self.pg2, pkt_count)
+        packets2 = self.create_stream(self.pg2, self.pg0, pkt_count)
+        # add the streams to the source interfaces
+        self.pg0.add_stream(packets0)
+        self.pg1.add_stream(packets1)
+        self.pg2.add_stream(packets2)
+        # enable capture on all interfaces
+        for pg in self.pg_interfaces:
+            pg.enable_capture()
+        # start the packet generator
+        self.pg_start()
+
+        # get captures
+        if_caps = []
+        for pg in [self.pg1, self.pg2]:  # we are expecting captures on pg1/pg2
+            if_caps.append(pg.get_capture())
+            for packet in if_caps[-1]:
+                try:
+                    self.logger.debug(ppp("SPD - Got packet:", packet))
+                except Exception:
+                    self.logger.error(
+                        ppp("Unexpected or invalid packet:", packet))
+                    raise
+        self.logger.debug("SPD: Num packets: %s", len(if_caps[0].res))
+        self.logger.debug("SPD: Num packets: %s", len(if_caps[1].res))
+
+        # verify captures that matched BYPASS rule
+        self.verify_capture(self.pg0, self.pg1, if_caps[0])
+        self.verify_capture(self.pg1, self.pg2, if_caps[1])
+        # verify that traffic to pg0 matched DISCARD rule and was dropped
+        self.pg0.assert_nothing_captured()
+        # verify all packets that were expected to match rules, matched
+        # pg0 -> pg1
+        self.verify_policy_match(pkt_count, policy_01)
+        self.verify_policy_match(0, policy_02)
+        # pg1 -> pg2
+        self.verify_policy_match(pkt_count, policy_11)
+        self.verify_policy_match(0, policy_12)
+        # pg2 -> pg0
+        self.verify_policy_match(0, policy_21)
+        self.verify_policy_match(pkt_count, policy_22)
+        # check that 3 matching policies in SPD have been cached
+        self.verify_num_outbound_flow_cache_entries(3)
+
+
+class IPSec4SpdTestCaseOverwriteStale(SpdFlowCacheOutbound):
+    """ IPSec/IPv4 outbound: Policy mode test case with flow cache \
+        (overwrite stale entries)"""
+    def test_ipsec_spd_outbound_overwrite(self):
+        # The operation of the flow cache is setup so that the entire cache
+        # is invalidated when adding or removing an SPD policy rule.
+        # For performance, old cache entries are not zero'd, but remain
+        # in the table as "stale" entries. If a flow matches a stale entry,
+        # and the epoch count does NOT match the current count, the entry
+        # is overwritten.
+        # In this test, 3 active rules are created and matched to enter
+        # them into the flow cache.
+        # A single entry is removed to invalidate the entire cache.
+        # We then readd the rule and test that overwriting of the previous
+        # stale entries occurs as expected, and that the flow cache entry
+        # counter is updated correctly.
+        self.create_interfaces(3)
+        pkt_count = 2
+        # bind SPD to all interfaces
+        self.spd_create_and_intf_add(1, self.pg_interfaces)
+        # add output rules on all interfaces
+        # pg0 -> pg1
+        policy_0 = self.spd_add_rem_policy(  # outbound
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        # pg1 -> pg2
+        policy_1 = self.spd_add_rem_policy(  # outbound
+            1, self.pg1, self.pg2, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        # pg2 -> pg0
+        policy_2 = self.spd_add_rem_policy(  # outbound
+            1, self.pg2, self.pg0, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="discard")
+
+        # check flow cache is empty (0 active elements) before sending traffic
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # create the packet streams
+        packets0 = self.create_stream(self.pg0, self.pg1, pkt_count)
+        packets1 = self.create_stream(self.pg1, self.pg2, pkt_count)
+        packets2 = self.create_stream(self.pg2, self.pg0, pkt_count)
+        # add the streams to the source interfaces
+        self.pg0.add_stream(packets0)
+        self.pg1.add_stream(packets1)
+        self.pg2.add_stream(packets2)
+        # enable capture on all interfaces
+        for pg in self.pg_interfaces:
+            pg.enable_capture()
+        # start the packet generator
+        self.pg_start()
+
+        # get captures from ifs
+        if_caps = []
+        for pg in [self.pg1, self.pg2]:  # we are expecting captures on pg1/pg2
+            if_caps.append(pg.get_capture())
+            for packet in if_caps[-1]:
+                try:
+                    self.logger.debug(ppp("SPD Add - Got packet:", packet))
+                except Exception:
+                    self.logger.error(
+                        ppp("Unexpected or invalid packet:", packet))
+                    raise
+
+        # verify captures that matched BYPASS rules
+        self.verify_capture(self.pg0, self.pg1, if_caps[0])
+        self.verify_capture(self.pg1, self.pg2, if_caps[1])
+        # verify that traffic to pg0 matched DISCARD rule and was dropped
+        self.pg0.assert_nothing_captured()
+        # verify all policies matched the expected number of times
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(pkt_count, policy_1)
+        self.verify_policy_match(pkt_count, policy_2)
+        # check flow/policy match was cached for: 3x output policies
+        self.verify_num_outbound_flow_cache_entries(3)
+
+        # adding an inbound policy should not invalidate output flow cache
+        self.spd_add_rem_policy(  # inbound
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=0, priority=10, policy_type="bypass")
+        # check flow cache counter has not been reset
+        self.verify_num_outbound_flow_cache_entries(3)
+
+        # remove a bypass policy - flow cache counter will be reset, and
+        # there will be 3x stale entries in flow cache
+        self.spd_add_rem_policy(  # outbound
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass",
+            remove=True)
+        # readd policy
+        policy_0 = self.spd_add_rem_policy(  # outbound
+            1, self.pg0, self.pg1, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        # check counter was reset with flow cache invalidation
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # resend the same packets
+        self.pg0.add_stream(packets0)
+        self.pg1.add_stream(packets1)
+        self.pg2.add_stream(packets2)
+        for pg in self.pg_interfaces:
+            pg.enable_capture()  # flush previous captures
+        self.pg_start()
+
+        # get captures from ifs
+        if_caps = []
+        for pg in [self.pg1, self.pg2]:  # we are expecting captures on pg1/pg2
+            if_caps.append(pg.get_capture())
+            for packet in if_caps[-1]:
+                try:
+                    self.logger.debug(ppp("SPD Add - Got packet:", packet))
+                except Exception:
+                    self.logger.error(
+                        ppp("Unexpected or invalid packet:", packet))
+                    raise
+
+        # verify captures that matched BYPASS rules
+        self.verify_capture(self.pg0, self.pg1, if_caps[0])
+        self.verify_capture(self.pg1, self.pg2, if_caps[1])
+        # verify that traffic to pg0 matched DISCARD rule and was dropped
+        self.pg0.assert_nothing_captured()
+        # verify all policies matched the expected number of times
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(pkt_count*2, policy_1)
+        self.verify_policy_match(pkt_count*2, policy_2)
+        # we are overwriting 3x stale entries - check flow cache counter
+        # is correct
+        self.verify_num_outbound_flow_cache_entries(3)
+
+
+class IPSec4SpdTestCaseCollision(SpdFlowCacheOutbound):
+    """ IPSec/IPv4 outbound: Policy mode test case with flow cache \
+        (hash collision)"""
+    # Override class setup to restrict vector size to 16 elements.
+    # This forces using only the lower 4 bits of the hash as a key,
+    # making hash collisions easy to find.
+    @classmethod
+    def setUpConstants(cls):
+        super(SpdFlowCacheOutbound, cls).setUpConstants()
+        cls.vpp_cmdline.extend(["ipsec", "{",
+                                "ipv4-outbound-spd-flow-cache on",
+                                "ipv4-outbound-spd-hash-buckets 16",
+                                "}"])
+        cls.logger.info("VPP modified cmdline is %s" % " "
+                        .join(cls.vpp_cmdline))
+
+    def test_ipsec_spd_outbound_collision(self):
+        # The flow cache operation is setup to overwrite an entry
+        # if a hash collision occurs.
+        # In this test, 2 packets are configured that result in a
+        # hash with the same lower 4 bits.
+        # After the first packet is received, there should be one
+        # active entry in the flow cache.
+        # After the second packet with the same lower 4 bit hash
+        # is received, this should overwrite the same entry.
+        # Therefore there will still be a total of one (1) entry,
+        # in the flow cache with two matching policies.
+        # crc32_supported() method is used to check cpu for crc32
+        # intrinsic support for hashing.
+        # If crc32 is not supported, we fall back to clib_xxhash()
+        self.create_interfaces(3)
+        pkt_count = 5
+        # bind SPD to all interfaces
+        self.spd_create_and_intf_add(1, self.pg_interfaces)
+        # add rules
+        policy_0 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg1, self.pg2, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+        policy_1 = self.spd_add_rem_policy(  # outbound, priority 10
+            1, self.pg2, self.pg0, socket.IPPROTO_UDP,
+            is_out=1, priority=10, policy_type="bypass")
+
+        # check flow cache is empty (0 active elements) before sending traffic
+        self.verify_num_outbound_flow_cache_entries(0)
+
+        # create the packet streams generating collision on last 4 bits
+        if self.crc32_supported():
+            # packet hashes to:
+            # 432c99c2
+            packets1 = self.create_stream(self.pg1, self.pg2, pkt_count, 1, 1)
+            # 31f8f3f2
+            packets2 = self.create_stream(self.pg2, self.pg0, pkt_count, 6, 6)
+        else:  # clib_xxhash
+            # ec3a258551bc0306
+            packets1 = self.create_stream(self.pg1, self.pg2, pkt_count, 2, 2)
+            # 61fee526d18d7a6
+            packets2 = self.create_stream(self.pg2, self.pg0, pkt_count, 3, 3)
+
+        # add the streams to the source interfaces
+        self.pg1.add_stream(packets1)
+        self.pg2.add_stream(packets2)
+        # enable capture on all interfaces
+        for pg in self.pg_interfaces:
+            pg.enable_capture()
+        # start the packet generator
+        self.pg_start()
+
+        # get captures from ifs - the proper pkt_count of packets was saved by
+        # create_packet_info() based on dst_if parameter
+        if_caps = []
+        for pg in [self.pg2, self.pg0]:  # we are expecting captures on pg2/pg0
+            if_caps.append(pg.get_capture())
+            for packet in if_caps[-1]:
+                try:
+                    self.logger.debug(ppp(
+                        "SPD - Got packet:", packet))
+                except Exception:
+                    self.logger.error(ppp(
+                        "Unexpected or invalid packet:", packet))
+                    raise
+        self.logger.debug("SPD: Num packets: %s", len(if_caps[0].res))
+        self.logger.debug("SPD: Num packets: %s", len(if_caps[1].res))
+
+        # verify captures that matched BYPASS rule
+        self.verify_capture(self.pg1, self.pg2, if_caps[0])
+        self.verify_capture(self.pg2, self.pg0, if_caps[1])
+        # verify all packets that were expected to match rules, matched
+        self.verify_policy_match(pkt_count, policy_0)
+        self.verify_policy_match(pkt_count, policy_1)
+        # we have matched 2 policies, but due to the hash collision
+        # one active entry is expected
+        self.verify_num_outbound_flow_cache_entries(1)
+
+
+if __name__ == '__main__':
+    unittest.main(testRunner=VppTestRunner)