harvey/sys/src/cmd/cec/Protocol

Coraid Ethernet Console (cec)

Abstract

The Coraid Ethernet Console (cec) implements a bidirectional conversation
over raw ethernet frames with provisions for retransmission and discovery.  
Like serial consoles, each machine has a single shared interface per service
type,  but any number connections can be made from any machine connected to 
the same physical network.  The communications from the various connections
will be interleaved.  The first implementation of cec is for console communications
to Coraid appliances.

Outline

1. Packet format
2. The Tdiscover packet and Toffer reply.
3.  Initializing a connection. Tinit[abc]
4.  The connection.  Tdata and Tack
5.  Closing the connection.  Treset

1. Packet Format

All cec packets are follow the layout implied by the following structure

	struct Pkt {
		uchar	dst[6];		// destination ethernet address
		uchar	src[6];		// source ethernet address
		uchar	etype[2];		// service type
		uchar	type;		// type of packet.
		uchar	conn;		// unique connection id.
		uchar	seq;		// packet sequence number
		uchar	len;		// data length.
		uchar	data[1500];
	};

The packet types are as follows:

	enum {
		Tinita,
		Tinitb,
		Tinitc,
		Tdata,
		Tack,
		Tdiscover,
		Toffer,
		Treset,
	};

2. The Tdiscover packet and Toffer reply.

The Tdiscover packet is used to discover the avaliable cec devices on the local
network.  The packet sent is of the form

	Tdiscover = {
	[dest]	0xffffffff,
	[src]	mac of local interface,
	[etype]	service type,
	[type]	Tdiscover,
	[seq]	0,
	[len]	0,
	[data]	0,
	}

The reply to the Tdiscover packet is of type Toffer which differes from
Tdiscover in that data and len may be set.  The contents of data is application
specific.

3.  Initializing a connection. Tinit[abc]

A connection is initialized by the following conversation: In addition
to the fields set for the Tdiscover packet, he client sends a packet
of type Tinita with the conntag of its choice.  The server responds to
Tinita with a packet of type Tinitb.  And finally the client sents a
Tinitc packet back to the server, completing the connection.

4.  The connection.  Tdata and Tack

Data is sent from the client to the console server with the Tdata packet.
The seq field is incremented for each data packet sent.  Thus data packets
may be transmitted if lost.  The data is whatever data the client has to
send to the server, up to 1500 bytes.  Typically, however, each keystroke
is sent in a seperate packet.  The len field is the length of the
data.

The server responds to a Tdata message with a Tack message with the 
same seq sequence number.

5.  Closing the connection.  Treset

Either the server of the client may send a Treset message to close the 
connection.  There is no response to a Treset message, however any
information associated with that connection (conntag) is discarded 
when the Treset message is recieved.