ref: 73862cb9cf6058273c3ce9c025bde55433306265
dir: /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, the 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 255 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.