# Address Resolution Protocol

This unit contains a struct pointing to [com/ip4]'s active frame payload as well
as a cache that store mappings received from ARP replies.

## API

### Structures

struct ARPRequest {
  beshort HRD PRO ;
  uchar HLN PLN ;
  beshort OP ;
  [void,0] SHA ;
}
addrof payloadptr ARPRequest bindstruct

This points to [com/link]'s active frame's payload.

Because SHA, SPA, THA and TPA are of variable length, the 3 last fields can't
be regular struct fields

### Constants

ARPCACHECNT  Maximum number of entries that the cache can contain.
ARPSZ        Size in bytes of a regular ethernet ARP request/reply

### Words

SPA
THA
TPA ( -- a )
  Yield the address of the 3 last fields of the struct, taking HLN and PLN in
  account.

.arpcache ( -- )
  Print all mapping stored in the ARP cache.

arpadd ( ipaddr macaddr -- )
  Add a new mapping to the ARP cache. Aborts if there's no more space.

?arpadd ( ipaddr macaddr -- )
  If ipaddr doesn't exist in cache, add mapping. Otherwise, update mapping with
  specified macaddr.

arplookup ( ipaddr -- macaddr-or-0 )
  Look for ipaddr in cache. If it exists, yield the associated mac address.
  Otherwise, yield 0.

arprequest ( ipaddr link -- )
  Send a ARP request for the specified IP4 addr on the specified link
  [com/link].

handlearp ( -- )
  Plugged into [com/net]'s incoming handlers, handle incoming ARP requests
  and reply. Requests are replied to if they're querying selfaddr [com/ip4].
  Replies are stored in the ARP cache.