ARP
[Base Protocols]

Collaboration diagram for ARP:

---

Detailed Description

RFC 826 address resolution protocol.

ARP is used to map IP addresses to hardware addresses. Each network interface of Nut/Net keeps its own mapping table.

When an IP packet has to be sent out, IP needs the hardware address to pass it to the Ethernet layer. If the mapping is not in the ARP cache, an Ethernet broadcast packet is sent to the local network requesting the physical hardware address for the given IP address.

Todo:

Add functions to manually add or remove ARP entries.

Todo:

Add function to query ARP tables.

Todo:

Response may reuse received ARP packet.

Data Structures
struct	_ARPENTRY
	ARP entry structure. More...
struct	_ARPENTRY
	ARP entry structure. More...
ARP Configuration
The Nut/OS Configurator may be used to override the default values.
#define	MAX_ARPAGE   9
	Maximum age of an entry in the ARP cache in minutes.
#define	MAX_ARPREQUESTS   1
	Maximum number of ARP requests generated per query.
#define	MIN_ARPWAIT   500
	Minimum wait before sending out a new ARP request.
Defines
#define	ATF_REM   0x01
	Entry marked for removal.
#define	ATF_COM   0x02
	Completed entry.
#define	ATF_PERM   0x04
	Permanent entry.
Typedefs
typedef _ARPENTRY	ARPENTRY
	ARP entry type.
Functions
void	NutArpCacheUpdate (NUTDEVICE *dev, u_long ip, u_char *ha)
	Update an ARP entry.
int	NutArpCacheQuery (NUTDEVICE *dev, CONST u_long ip, u_char *mac)
	Query MAC address for a specified IP address.
void	NutArpInput (NUTDEVICE *dev, NETBUF *nb)
	Handle incoming ARP packets.
NETBUF *	NutArpAllocNetBuf (u_short type, u_long ip, u_char *mac)
	Allocate an ARP network buffer structure.
int	NutArpOutput (NUTDEVICE *dev, NETBUF *nb)
	Send an ARP packet.

---

Define Documentation

#define MAX_ARPAGE   9

Maximum age of an entry in the ARP cache in minutes. Outdated entries will be regularly removed, forcing the Ethernet interface to generate new ARP requests. This way MAC address changes are detected.

#define MAX_ARPREQUESTS   1

Maximum number of ARP requests generated per query. If no ARP response is received after sending out the specified number of request, the related IP address is considered unreachable.

#define MIN_ARPWAIT   500

Minimum wait before sending out a new ARP request. The specified number of milliseconds will be doubled on each retry.

---

Function Documentation

void NutArpCacheUpdate (  NUTDEVICE *  dev, u_long  ip, u_char *  ha )

Update an ARP entry. If an entry with the same IP address exists, then this entry is updated. If no entry exists, a new one is created. All threads waiting for address resolution are woken up. Note: This function is automatically called on each incoming ARP telegram. Applications typically do not call this function. Parameters: dev  Identifies the device. ip  Requested IP address in network byte order. ha  Pointer to a buffer which receives the MAC address.

int NutArpCacheQuery (  NUTDEVICE *  dev, CONST u_long  ip, u_char *  mac )

Query MAC address for a specified IP address. If no entry is available in the ARP cache, an incomplete entry is created and ARP requests are generated on increasing time intervals. The calling thread is suspended until a matching ARP response is received or until a number of requests have been generated without receiving a response. Note: This function is automatically called on each outgoing IP packet. Applications typically do not call this function. Parameters: dev  Identifies the device. ip  IP address of which the caller asked the MAC address. mac  Buffer for the retrieved MAC address. Returns: 0 if address resolved, -1 otherwise.

void NutArpInput (  NUTDEVICE *  dev, NETBUF *  nb )

Handle incoming ARP packets. Packets not destined to us or packets with unsupported address type or item length are silently discarded. Note: This routine is called by the Ethernet layer on incoming ARP packets. Applications typically do not call this function. Parameters: dev  Identifies the device that received the packet. nb  Pointer to a network buffer structure containing the ARP packet.

NETBUF* NutArpAllocNetBuf (  u_short  type, u_long  ip, u_char *  mac )

Allocate an ARP network buffer structure. Parameters: type  Type of ARP packet. ip  Target IP address. mac  Target MAC address, null pointer for broadcast. Returns: Pointer to the allocated network buffer structure or 0 on failure.

int NutArpOutput (  NUTDEVICE *  dev, NETBUF *  nb )

Send an ARP packet. Note: Applications typically do not call this function. Parameters: dev  Identifies the device to use. nb  Network buffer structure containing the packet to be sent. The structure must have been allocated by a previous call NutNetBufAlloc(). Returns: 0 on success, -1 in case of any errors.

---