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|
emcast protocol
by David Helder
Copyright (C) 2001 Regents of the University of Michigan
============================================================
A program is compiled with libemcast. Libemcast communicates with an
emcast handler using the emcast protocol over two streams, the control
stream and the data stream. The control stream is two-way and the
data stream one-way from handler to libemcast. See the diagram below.
For example, the handler might be a child process, the control stream
two pipes, and the data stream a FIFO.
+---------------------+ stream-ctrl +---------+
| program | libemcast | <----> | handler |
+---------------------+ <---- +---------+
stream-data
Libemcast sends requests and receives responses from the handler using
the control stream. The handler sends requests using the data stream.
The handler can not receive responses using the data stream. If only
one stream were used and both libemcast and the handler sent a
request, each would think the other's request was a response to its
own request, and a malfunction may occur. Using two streams seems the
best solution.
A short is two bytes. A long is four bytes. All integers are in
network byte order.
A message summary is below. INIT, JOIN, LEAVE, SEND, GETOPT, and
SETOPT are sent from libemcast to the handler over the control stream.
RECV is sent from the handler to libemcast over the data stream.
INIT
----
libemcast sends an INIT message after the connection is established.
This provides basic version negotiation.
libemcast -> handler
INIT 0x0000 short
VERSION 0x0001 short
FIFO_LENGTH short
FIFO FIFO_LENGTH bytes
handler <- libemcast
RESPONSE 0 on success byte
non-0 on falure
VERSION 0x0001 short
libemcast sends the highest version it supports. handler returns
highest version it supports and must be less than or equal to the
highest version libemcast supports. The version the handler returns
is the version used. This document describes version 1.
The FIFO is created by libemcast. Each side will open it for reading
and writing after the initialization sequence.
[If we were to support the protocol over sockets, this would need to
change.]
JOIN
----
libemcast sends a JOIN message to join a multicast group. A group is
either created or joined. Only one group can be created or joined.
libemcast -> handler
ID 0x0001 or 0x0002 short
URL_LENGTH length of URL unsigned short
URL URL URL_LENGTH bytes
hander -> libemcast
RESPONSE 0 on success byte
non-0 on failure
LEAVE
-----
libemcast sends a LEAVE message to leave the group. libemcast must
close the connection after receiving the response. The handler must
close the connection after sending the response.
libemcast -> handler
ID 0x0003 short
handler -> libemcast
RESPONSE 0 on success byte
non-0 on failure
SEND
----
libemcast send a SEND message to send multicast data. Delivery is
assumed to be best-effort. The handler should return a response
immediately.
libemcast -> handler
ID 0x0005 short
LENGTH length of DATA unsigned short
DATA data LENGTH bytes
handler -> libemcast
RESPONSE 0 on success byte
non-0 on failure
RECV
----
handler -> libemcast
ID 0x0006 short
LENGTH length of DATA unsigned short
FROM_LEN length of FROM unsigned short
DATA data LENGTH bytes
FROM from FROM_LEN bytes
RECV's are sent over the second stream. libemcast does not respond to
the handler.
GETOPT
------
libemcast -> handler
ID 0x0007 short
NAME_LEN length of NAME unsigned short
NAME name of option NAME_LEN bytes
handler -> libemcast
RESPONSE 0 on success, byte
1 if bad option
X on failure
VAL_LEN length of VAL unsigned short
VAL value of option VAL_LEN bytes
VAL_LEN/VAL isn't sent if RESPONSE is non-zero.
SETOPT
------
libemcast -> handler
ID 0x0007 short
NAME_LEN length of NAME unsigned short
VAL_LEN length of VAL unsigned short
NAME name of option NAME_LEN bytes
VAL value of option VAL_LEN bytes
handler -> libemcast
RESPONSE 0 on success, byte
1 if bad option
2 if bad value
|
