# Ethernet com link

This unit implements an Ethernet [com/link] that primarily deals with ethernet
frames:

6b Destination MAC
6b Source MAC
2b Ethertype or payload size
.. Payload

Optional 802.1Q VLAN tagging (Ethertype $8100) is not handled yet.

## Ethernet and ARP

Frames need to be sent to particular MAC addresses, which we don't know
beforehand. The ARP protocol [com/arp] is designed to solve this problem by
mapping IP [com/ip4] addresses to MAC addresses.

The proper way to proceed would be, that whenever we want to send a frame
containing an IP datagram where the destination IP is not in the ARP cache,
we hold this frame for a while, do an ARP request and, when it returns, send the
frame we wanted to send earlier to the proper MAC address.

The problem is that we definitely don't want to do this in a blocking way. This
would make the system freeze for as long as it takes for a ARP reply to come
back, if it does.

We need to do this in an asynchronous manner, but this involves a quite complex
system out outgoing buffer queues.

The approach we take in Dusk is one of slight network rudeness: send ethernet
frames to ethernet broadcast address (FF:FF:FF:FF:FF:FF) as long as the ARP
request hasn't returned. This saves us the trouble of having to manage outgoing
buffer queues. This situation will typically be very short lived, so the network
spurious chatter should be minimal.

## API

### EtherLink struct

extends Link struct EtherLink {
  *uchar macptr ;
  method rxbuf txbuf ;
}

macptr  Pointer to the MAC address of this ethernet link

rxbuf ( link -- ?a u-or-0 )
  Check for an incoming frame and yield its range if there is one. Otherwise,
  u=0 and a is absent.

txbuf ( link -- a u )
  Find a free outbound buffer and yield its range. As per [com/link]
  specifications, it must be at least $200 bytes in size.

### ivalmap of an ethernet frame

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

addrof frameptr ivalmap {
  [uchar,6] dstmac srcmac ;
  beshort ethertype ;
}

### Constants

BROADCAST  Address to a FF:FF:FF:FF:FF:FF MAC address.

### Words

.mac ( a -- )
  Print a MAC address living at "a".

.ether ( -- )
  Pretty-print the active ethernet frame

macmove ( src dst -- )
  Copy MAC address from src to dst.

newetherlink ( sendframext txbufxt rxbufxt macptr -- link )
  Create a new [com/link] from the specified method XTs as well as a pointer to
  this link's MAC address.

  This unit takes care of implementing "readframe" and "beginframe" and
  "replytoframe".