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diff --git a/src/plugins/nat/nat_affinity.h b/src/plugins/nat/nat_affinity.h
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+/*
+ * Copyright (c) 2018 Cisco and/or its affiliates.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+/**
+ * @file
+ * @brief NAT plugin client-IP based session affinity for load-balancing
+ */
+
+#ifndef __included_nat_affinity_h__
+#define __included_nat_affinity_h__
+
+#include <vnet/ip/ip.h>
+#include <vppinfra/bihash_16_8.h>
+#include <vppinfra/dlist.h>
+
+typedef struct
+{
+  union
+  {
+    struct
+    {
+      ip4_address_t service_addr;
+      ip4_address_t client_addr;
+      /* align by making this 4 octets even though its a 1 octet field */
+      u32 proto;
+      /* align by making this 4 octets even though its a 2 octets field */
+      u32 service_port;
+    };
+    u64 as_u64[2];
+  };
+} nat_affinity_key_t;
+
+/* *INDENT-OFF* */
+typedef CLIB_PACKED(struct
+{
+  nat_affinity_key_t key;
+  u32 sticky_time;
+  u32 ref_cnt;
+  u32 per_service_index;
+  u8 backend_index;
+  f64 expire;
+}) nat_affinity_t;
+/* *INDENT-ON* */
+
+typedef struct
+{
+  nat_affinity_t *affinity_pool;
+  clib_bihash_16_8_t affinity_hash;
+  clib_spinlock_t affinity_lock;
+  dlist_elt_t *list_pool;
+  vlib_main_t *vlib_main;
+} nat_affinity_main_t;
+
+extern nat_affinity_main_t nat_affinity_main;
+
+/**
+ * @brief Get new affinity per service list head index.
+ *
+ * @returns new affinity per service list head index.
+ */
+u32 nat_affinity_get_per_service_list_head_index (void);
+
+/**
+ * @brief Flush all service affinity data.
+ *
+ * @param affinity_per_service_list_head_index Per sevice list head index.
+ */
+void nat_affinity_flush_service (u32 affinity_per_service_list_head_index);
+
+/**
+ * @brief Initialize NAT client-IP based affinity.
+ *
+ * @param vm vlib main.
+ *
+ * @return error code.
+ */
+clib_error_t *nat_affinity_init (vlib_main_t * vm);
+
+/**
+ * @brief Find service backend index for client-IP and take a reference
+ *  counting lock.
+ *
+ * @param client_addr Client IP address.
+ * @param service_addr Service IP address.
+ * @param proto IP protocol number.
+ * @param service_port Service L4 port number.
+ * @param backend_index Service backend index for client-IP if found.
+ *
+ * @return 0 on success, non-zero value otherwise.
+ */
+int nat_affinity_find_and_lock (ip4_address_t client_addr,
+				ip4_address_t service_addr, u8 proto,
+				u16 service_port, u8 * backend_index);
+
+/**
+ * @brief Create affinity record and take reference counting lock.
+ * @param client_addr Client IP address.
+ * @param service_addr Service IP address.
+ * @param proto IP protocol number.
+ * @param service_port Service L4 port number.
+ * @param backend_index Service backend index for client-IP.
+ * @param sticky_time Affinity sticky time in seconds.
+ * @param affinity_per_service_list_head_index Per sevice list head index.
+ *
+ * @return 0 on success, non-zero value otherwise.
+ */
+int nat_affinity_create_and_lock (ip4_address_t client_addr,
+				  ip4_address_t service_addr, u8 proto,
+				  u16 service_port, u8 backend_index,
+				  u32 sticky_time,
+				  u32 affinity_per_service_list_head_index);
+/**
+ * @brief Release a reference counting lock for affinity.
+ *
+ * @param client_addr Client IP address.
+ * @param service_addr Service IP address.
+ * @param proto IP protocol number.
+ */
+void nat_affinity_unlock (ip4_address_t client_addr,
+			  ip4_address_t service_addr, u8 proto,
+			  u16 service_port);
+
+#endif /* __included_nat_affinity_h__ */
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */