轉-Multicast server and client in Python

FROM:http://chaos.weblogs.us/archives/164
Python has complete support for sockets. Some of the APIs though have a different signature than the POSIX equivalents. It is therefore possible to write multicast servers and clients in Python. Multicasting is the ability to send data to select set of hosts. Broadcasting in a network is to send data to all the hosts, unicast is when data is transfered between two hosts (typical one-one communication). Multicasting is the ability to send data to a multicast address and the clients are provided the data. Now that an introduction to multicasting is complete, lets look at how to write a simple multicast server and a multicast client.
Multicast servers are written very similar to a standard unicast server, its the clients who have a slightly different code. The major change is made in setting socket options on the socket transmitting and receiving data.

The server code looks like this:
################################################################
import socket, time

ANY = "0.0.0.0"
SENDERPORT = 1501
MCAST_ADDR = "224.168.2.9"
MCAST_PORT = 1600

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
#The sender is bound on (0.0.0.0:1501)
sock.bind((ANY,SENDERPORT))
#Tell the kernel that we want to multicast and that the data is sent to everyone (255 is the level of multicasting)
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 255)
while 1:
    time.sleep(10)
    #send the data "hello, world" to the multicast addr: port
    #Any subscribers to the multicast address will receive this data
    sock.sendto("Hello World", (MCAST_ADDR,MCAST_PORT));
################################################################
The TTL field of 255 indicates that the data is unrestricted in scope.

The corresponding client code to receive the data being transmitted by the sender will look like this:
################################################################
import socket, time

ANY = "0.0.0.0"
MCAST_ADDR = "224.168.2.9"
MCAST_PORT = 1600
#create a UDP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
#allow multiple sockets to use the same PORT number
sock.setsockopt(socket.SOL_SOCKET,socket.SO_REUSEADDR,1)
#Bind to the port that we know will receive multicast data
sock.bind((ANY,MCAST_PORT))
#tell the kernel that we are a multicast socket
sock.setsockopt(socket.IPPROTO_IP, socket.IP_MULTICAST_TTL, 255)
#Tell the kernel that we want to add ourselves to a multicast group
#The address for the multicast group is the third param
status = sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP,
    socket.inet_aton(MCAST_ADDR) + socket.inet_aton(ANY));

sock.setblocking(0)
ts = time.time()
while 1:
    try:
        data, addr = sock.recvfrom(1024)
    except socket.error, e:
        pass
    else:
        print "We got data!"
        print "FROM: ", addr
        print "DATA: ", data
################################################################
The import part of the client code is the setsockopt for multicast socket and to add the socket to a multicast group (socket.IP_ADD_MEMBERSHIP). Also note that the socket is bound to ANY:MCAST_PORT. The server sends data to the multicast address and on the client side, the socket that is bound to the multicast port subscribes to the multicast group and receives data on the MCAST_PORT. It is important the client code bind itself to the MCAST_PORT to receive data from the multicast address. Another important thing to note is the MCAST_ADDR. This address should be a valid multicast address, and not a unicast (localhost / INADDR_ANY / any valid IP).

TTL     Scope
----------------------------------------------------------------------
   0    Restricted to the same host. Won't be output by any interface.
   1    Restricted to the same subnet. Won't be forwarded by a router. <If not otherwise specified, multicast datagrams are sent with a default value of 1, to prevent them to be forwarded beyond the local network.
 <32         Restricted to the same site, organization or department.
 <64 Restricted to the same region.
<128 Restricted to the same continent.
<255 Unrestricted in scope. Global.

224.0.1.0到238.255.255.255爲用戶可用的組播地址，在全網範圍內有效。其中232.0.0.0/8爲SSM組地址，而其他則屬於ASM組地址。

http://www.h3c.com.cn/Products___Technology/Technology/Group_Management/Other_technology/Technology_book/200803/336048_30003_0.htm
根據接收者對組播源處理方式的不一樣，組播模型分爲如下兩大類：
ASM模型：即任意源組播模型。在ASM模型中，任一發送者均可做爲組播源向某組播組地址發送組播信息，接收者經過加入由該組播組地址標識的組播組以得到發往該組播組的組播信息。在ASM模型中，接收者沒法預先知道組播源的位置，但能夠在任意時間加入或離開組播組。
SSM模型：即指定信源組播模型。在現實生活中，用戶可能只對某些組播源發送的組播信息感興趣，而不肯接收其它源發送的信息。SSM模型爲用戶提供了一種可以在客戶端指定組播源的傳輸服務。