负载均衡器

负载均衡器的百度解释：
    负载均衡（LoadBalance）建立在现有网络结构之上，它提供了一种廉价、有效、透明的方法，来扩展网络设备和服务器的带宽、增加吞吐量、加强网络数据处理能力、提高网络的灵活性和可用性。

    有名的硬件：F5 BIG-IP(SNAT,NPATH)(3-7层)
    有名的开源软件：LVS(Dr,Tun,NAT)(3-4层)，haproxy(NAT)(7层)，nginx(NAT)(3-7层)
   
   
NAT:   
When a user accesses the service provided by the server cluster, the request packet destined for virtual IP address (the external IP address for the load balancer) arrives at the load balancer. The load balancer examines the packet's destination address and port number. If they are matched for a virtual server service according to the virtual server rule table, a real server is chosen from the cluster by a scheduling algorithm, and the connection is added into the hash table which record the established connection. Then, the destination address and the port of the packet are rewritten to those of the chosen server, and the packet is forwarded to the server. When the incoming packet belongs to this connection and the chosen server can be found in the hash table, the packet will be rewritten and forwarded to the chosen server. When the reply packets come back, the load balancer rewrites the source address and port of the packets to those of the virtual service. After the connection terminates or timeouts, the connection record will be removed in the hash table.
Packet rewriting works as follows.

The incoming packet for web service would has source and destination addresses as:
SOURCE     202.100.1.2:3456     DEST     202.103.106.5:80

The load balancer will choose a real server, e.g. 172.16.0.3:8000. The packet would be rewritten and forwarded to the server as:
SOURCE     202.100.1.2:3456     DEST     172.16.0.3:8000

Replies get back to the load balancer as:
SOURCE     172.16.0.3:8000     DEST     202.100.1.2:3456

The packets would be written back to the virtual server address and returned to the client as:
SOURCE     202.103.106.5:80     DEST     202.100.1.2:3456

NPATH(DR)： 
When a user accesses a virtual service provided by the server cluster, the packet destined for virtual IP address (the IP address for the virtual server) arrives. The load balancer(LinuxDirector) examines the packet's destination address and port. If they are matched for a virtual service, a real server is chosen from the cluster by a scheduling algorithm, and the connection is added into the hash table which records connections. Then, the load balancer directly forwards it to the chosen server. When the incoming packet belongs to this connection and the chosen server can be found in the hash table, the packet will be again directly routed to the server. When the server receives the forwarded packet, the server finds that the packet is for the address on its alias interface or for a local socket, so it processes the request and return the result directly to the user finally. After a connection terminates or timeouts, the connection record will be removed from the hash table.

==over==