In order to use multicast to transfer file crossovers network segments in WebGIS system

every gateway has to open IGMP protocol

which unavoidably leads to hidden security trouble such as network storm

With using TCP to transfer file

server has to send data to clients in turn

which decreases efficiency and engrosses more band width.To solve this problem

MAP（multicast agent protocol） was established to transfer data.The MAP dynamically selected one multicast agent in every subnet.When an agent received data from server via TCP protocol

it multicast files into local subnet without opening IGMP protocol.By this way multicast could play such advantages

without opening IGMP router protocol.The key steps are as follows:Firstly

every client joins the p2p network

and gets the newest resources from their friends to make resource simultaneously.In the next place

client looks up the multicast agent and takes part in the agent election.If wins

the agent connects the server to get the newest resource.And then

the server of WebGIS transfers the resource to multicast agent in turn by TCP protocol.The agent multicasts the resource in local segment.At last

if there is losing package

it will be compensated by the p2p network.Then the update task of all the clients is finished quickly.The load of the multicast was greater than common nodes.When agent finded that there was one client whose performance was better or quited the system normally

it demised the privilege of agent to more suitable node.The parameters of performance included the value of CPU

the size of memory and the available space of hard disk.When an agent quited abnormally

its connected nodes would receive the FD

C

LOSE message

then these nodes would launch the campaign of multicast election in local segment.However

there was one special situation that agent and its connected nodes quited almost at the same time abnormally

if that happened

the campaign couldn’t be executed.This problem can be solved in this way that every node who sent node to quit sended a message to search agent in p2p network.If failed

the node launches the campaign of multicast election activity.Due to multicast unreliability

some clients would lose the packet or quit abnormally or when it logined in the system the agent began to translate data.All of these situation would lead some clients to lack packets.Consequently in every subnet segment p2p nework was constructed to compensate packet

which lightened the burden of multicast proxy greatly.After logined in system

each client sent message to local segment by multicast to look up friend.When the clients in the same local segment received this message

they checked the number of their friend and then they would connect them if the number was less than three

otherwise reject this message.And this client only received three relative request.For this client

if the waiting time was out

it meaned that it was the first node in local segment

then it was the multicast agent.It should connect the WebGIS server immediately.In this way

the local p2p network was constructed.The shape of the network was hexagon.Every peer had three friends and there only was one supernode who managed the p2p network.Its task was to calculate information and update data and so on.Generally that was multicast.In order to increase the server’s load and in time accept client’s connection

completion port was used to manage socket connection between server and client.To improve the transfer efficiency

thread pool was adopted to response toclients’ on-line request and memory pool was used to manage resource which was allocated to restore clients’ information.We compare MAP with TCP by abundant and exact experiments.The results shows we find that there is a linear relation between the number of segment and the time of transfer in MAP protocol.It has nothing to do with the number of clients in subnet

which demonstrates that MAP exceeds TCP greatly.Additionally

metadata was used to converse among server、 multicast proxy andcommon clients

which can support to transfer files from broken dot.