[[Title:Econet over IP]]
[[Category:Networking]]
===Econet Packet===
Econet packets can be ecapsulated in UDP (User Datagram Protocol) packets to
be transmitted in IP (Internet protocol) packets to be transmitted over a
network with IP protocol, such as Ethernet, as done by Acorn Universal
Networking.

An [[Econet packet]] looks like this:

  +------+------+-----+-----+---------+------+-----------------------------+
  | Dest | Dest | Src | Src | Control | Port |         Data                |
  | Stn  | Net  | Stn | Net |  Byte   |      |                             |
  +------+------+-----+-----+---------+------+-----------------------------+
   <-------- - - Packet Header - - ---------> <--- - - Packet Data - - --->

The Econet system sends a packet in four frames using a four-way handshake
which the 6854 ADLC puts on the wire as follows:

  Send:    Scout frame with packet header
  Receive: Scout fcknowledge frame
  Send:    Data frame with packet data
  Receive: Data acknowledge frame

See [[Econet packet]].

An Econet packet is sent over IP with a two-way handshake, sending a UDP
data packet and waiting for an acknowledge or a negative-acknowledge UDP
packet.

===UDP packet===
The UDP data contains an eight-byte UDP header containing information from
the Econet packet header, followed by the variable-length UDP data which
contains eight bytes containing more information from the Econet packet
header followed by the Econet packet data:
  
    +-----------------------+
    |UDP Header:            |
    |  Source Port          | 16-bit port
    |  Dest Port            | &8000 for Econet-Over-IP
    |  Length of UDP Data   | 16-bit UDP data packet length
    |  Checksum             | 16-bit UDC checksum
    +-----------------------+
    |UDP Data:              |
    |  Transaction Type     | &01=Broadcast &02=Data packet, &03=ACK, &04=NAK,
    |                       | &05=Immediate, &06 Imm.Reply
    |  Econet Port          | From Econet packet header
    |  Econet Control Byte  | From Econet packet header with bit 7 cleared
    |  Padding &00          |      The Control Byte gets bit 7 set in the receive block
    |  Sequence Number      | This 32-bit number increments by 4 on each transaction
    |                       |  Acks and Naks quote back the sequence they relate to
    | --------------------- |
    |        Econet         |
    |        Packet         |
    |         Data          |
    +-----------------------+

===IP Packet===
The UDP packet is encapsulated in an IP packet as follows:

    IP packet:
    +-------------------+
    | IP Header:        |
    |  Version          |
    |  Header Length    |
    |  Type             |
    |  Total Length     |
    |  ID               |
    |  Fragment         |
    |  Protocol         |
    |  Header CRC       |
    |  Source Address   | - site.network.net.stn
    |  Dest Address     | - site.network.net.stn
    |  Options          |
    | ----------------- |
    | IP Data:          |
    |  UDP packet       |
    | +---------------+ |
    | | UDP Header:   | |
    | |  Source Port  | |
    | |  Dest Port    | | - &8000 for Econet-Over-IP
    | |  Length       | |
    | |  Checksum     | |
    | | ------------- | |
    | | UDP Data:     | |
    | | +-----------+ | |
    | | | Tr.Type   | | | &01=Broadcast, &02=Data packet, &03=ACK, &04=NAK,
    | | |           | | | &05=Immediate, &06=Imm.reply
    | | | Eco Port  | | | From Econet packet header
    | | | Eco Ctrl  | | | From Econet packet header
    | | | Padding   | | |
    | | | Sequence  | | |
    | | | --------- | | |
    | | |  Econet   | | |
    | | |  Packet   | | |
    | | |   Data    | | |
    | | +-----------+ | |
    | +---------------+ |
    +-------------------+

===Broadcasts===
Broadcasts just send a data block and do not wait for any acknowledgement.

===Immediate Operations===
The only immediate operation implemented in Acorn Universal Networking
Econet over IP is MachinePeek to determine if a remote machine is present.
However, immediate operations do fit into the protocol.

===Coding===
The [http://mdfs.net/blib/ Net_Tx()] function in the Net library would
essentially do something like the following:

  DEFFNNet_Tx(Stn%,Ctrl%,Port%,Addr%,Len%,RAddr%):try=10:delay=50
  
  REPEAT
                             /* Prepare to send */
    sock=socket(AF_INET, SOCK_DGRAM, 0);
  
    name.sin_family=AF_INET;  /* Internet socket */
    name.sin_port=&8000;      /* Port for Econet-Over-Ethernet */
    name.sin_addr.s_addr=Stn% /* Destination */
  
    header[0]=&02             /* Econet header */
    header[1]=Port%
    header[2]=Ctrl%
    header[3]=0
    header[4..7]=RAddr%
  
    iov[0].base=&header       /* Point to header to send */
    iov[0].len=8              /* Eight bytes in header   */
    iov[1].base=Addr%         /* Point to data to send   */
    iov[1].len=Len%           /* Specified bytes of data */
  
    socketwritev(sock, &iov, 1);
    socketclose(sock);
  
                             /* Now listen for an acknowledgement */
    sock=socket(AF_INET, SOCK_DGRAM, 0);
  
    name.sin_family=AF_INET;  /* Internet socket */
    name.sin_port=&8000       /* Port for Econet-Over-Ethernet */
    name.sin_addr.s_addr=Stn% /* Who we're expecting an Ack from */
    bind(sock, &name, sizeof(name));
  
    socketread(sock, buf, buflen)
    socketclose(sock)
  
    IF buf[0]<>3 THEN try=try-1:IF try>0 THEN PROCdelay(delaytime)
  UNTIL buf[0]=3 OR try<1
  IF buf[0]=3 THEN =0  :REM Ok
  IF buf[0]=4 THEN =&42:REM Not listening
  =&40:REM Network error

===IP over Econet===
An IP packet can be encapsulated in an Econet packet as follows:

  +------+------+-----+-----+------+------+---------------+
  | Dest | Dest | Src | Src | Ctrl | Port | +-----------+ |
  | Stn  | Net  | Stn | Net | Byte | &D2  | | IP Packet | |
  |      |      |     |     |      |      | +-----------+ |
  +------+------+-----+-----+------+------+---------------+

The Control byte determines the type of IP packet:

  &81 IP
  &A1 ARP_REQUEST
  &A2 ARP_REPLY
  &8F IPBCAST_REQUEST
  &8E IPBCAST_REPLY

[[User:Jgharston|Jgharston]] 22:21, 14 May 2009 (UTC)