Added option LWIP_NETIF_HWADDRHINT (default=off) to cache ARP table indices with each pcb instead of single-entry cache for the complete stack.

src/include/ipv4/lwip/ip.h

@@ -83,7 +83,9 @@ err_t ip_output_if(struct pbuf *p, struct ip_addr *src, struct ip_addr *dest,
    /* Type Of Service */ \
   u8_t tos;              \
   /* Time To Live */     \
-  u8_t ttl
+  u8_t ttl;              \
+  /* link layer address resolution hint */ \
+  u8_t addr_hint
 
 /*
  * Option flags per-socket. These are the same like SO_XXX.

src/include/ipv6/lwip/ip.h

@@ -69,7 +69,9 @@ extern "C" {
    /* Type Of Service */ \
   u8_t tos;              \
   /* Time To Live */     \
-  u8_t ttl
+  u8_t ttl;              \
+  /* link layer address resolution hint */ \
+  u8_t addr_hint
 
 
 /* The IPv6 header. */

src/include/lwip/netif.h

@@ -155,6 +155,9 @@ struct netif {
   /* This function could be called to add or delete a entry in the multicast filter table of the ethernet MAC.*/
   err_t (*igmp_mac_filter)( struct netif *netif, struct ip_addr *group, u8_t action);
 #endif /* LWIP_IGMP */
+#if LWIP_NETIF_HWADDRHINT
+  u8_t *addr_hint;
+#endif /* LWIP_NETIF_HWADDRHINT */
 };
 
 #if LWIP_SNMP

src/include/lwip/opt.h

@@ -481,6 +481,15 @@
 #define LWIP_NETIF_CALLBACK             0
 #endif
 
+/** Cache link-layer-address hints (e.g. table indices) in struct netif.
+ TCP and UDP can make use of this to prevent scanning the ARP table
+ for every sent packet. While this is faster for big ARP tables or many
+ concurrent connections, it might be contra-productive if having a tiny
+ ARP table only or there never are concurrent connections. */
+#ifndef LWIP_NETIF_HWADDRHINT
+#define LWIP_NETIF_HWADDRHINT           0
+#endif
+
 /* Support loop interface (127.0.0.1) */
 #ifndef LWIP_HAVE_LOOPIF
 #define LWIP_HAVE_LOOPIF                0

src/include/netif/etharp.h

@@ -157,6 +157,11 @@ err_t etharp_raw(struct netif *netif, const struct eth_addr *ethsrc_addr,
 
 #define eth_addr_cmp(addr1, addr2) (memcmp((addr1)->addr, (addr2)->addr, ETHARP_HWADDR_LEN) == 0)
 
+/* finally, check some defines */
+#if ARP_TABLE_SIZE > 0x7f
+#error ARP_TABLE_SIZE must fit in an s8_t
+#endif
+
 #ifdef __cplusplus
 }
 #endif