Data communication method, data communication system and program

1. A data communication method comprising the steps of:

broadcasting data to a plurality of receivers;

calculating a maximum message length and a backoff time of reply messages to be returned from the receivers in response to the broadcasted data, wherein a total amount of data of the reply messages from all the receivers is predetermined, then the maximum message length is calculated by dividing the total amount of data of the reply messages from all the receivers by a number of the receivers, and the backoff time is calculated by dividing the total amount of data of the reply messages from all the receivers by a bandwidth of a communication route, wherein the total amount of data of the reply messages minus lengths of their headers is calculated from a number of broadcasted blocks of the data and wherein each reply message is a receipt acknowledgement message or a retransmission request message, the maximum message length Lrm is calculated based on the following equation

Lrm=Lh +F ( Nsb ) ×MLnac/Nr , and

the backoff time Tbo is calculated based on the following equation

Tbo =Lrm ×Nr/BW,

in which Lh is a header length of the reply message, F is a function defining an upper limit of a number of blocks for retransmission with a variable Nsb, Nsb is the number of the broadcasted blocks, MLnac is an average message length of a retransmission request per block, Nr is the number of the receivers, and BW is the bandwidth of the communication route;

sending the maximum message length and the backoff time to the receivers; and

receiving from respective receivers respective reply messages sent at any time within the backoff time and having a length equal to or less than the maximum message length.

2. The data communication method according to claim 1 , wherein

F ( Nsb )= C×Nsb

in which C is a constant.

3. The data communication method according to claim 1 , wherein

an allowable message length with a maximum of Lpmax and a minimum of Lpmin is defined for the reply messages,

the maximum message length Lrm is modified as Lrm =Lpmax if Lrm is above Lpmax,

the maximum message length Lrm is modified as Lrm =Lpmin if Lrm is below Lpmin, and

the maximum message length Lrm is modified as Lrm =Lworst if Lrm is above the message length Lworst, which is a longest reply message length among combinations of lost blocks of the data.

4. The data communication method according to claim 1 , wherein

the reply message includes a retransmission request message, and

the retransmission request message includes retransmission requests only for part of unreceived portions of the data.

5. The data communication method according to claim 4 , wherein the retransmission requests are made for blocks extracted at random from the unreceived portions of the data.

6. A data communication method comprising the steps of:

receiving data;

receiving a maximum message length and a backoff time of a reply message to be returned in response to the receipt of the data, wherein a total amount of data of the reply messages from all the receivers is predetermined, then the maximum message length is calculated by dividing the total amount of data of the reply messages from all receivers by a number of receivers, and the backoff time is calculated by dividing the total amount of data of the reply messages from all the receivers by a bandwidth of a communication route, wherein the total amount of data of the reply messages minus lengths of their headers is calculated from a number of broadcasted blocks of the data and wherein each reply message is a receipt acknowledgement message or a retransmission request message, the maximum message length Lrm is calculated based on the following equation

Lrm =Lh +F ( Nsb ) × MLnac/Nr , and

the backoff time Tbo is calculated based on the following equation

Tbo=Lrm×Nr/BW,

in which Lh is a header length of the reply message, F is a function defining an upper limit of a number of blocks for retransmission with a variable Nsb, Nsb is the number of the broadcasted blocks, MLnac is an average message length of a retransmission request per block, Nr is the number of the receivers, and BW is the bandwidth of the communication route;

generating the reply message having a length equal to or less than the maximum message length for blocks that could not be received in the data;

generating a transmission timing for the reply message at random within the backoff time; and

transmitting the reply message with the transmission timing.

7. The data communication method according to claim 6 , wherein blocks for which retransmission requests are to be made in the reply message are extracted at random from the blocks that could not be received.

8. The data communication method according to claim 1 , wherein

blocks of the data transmitted or received are clusters including original blocks into which the data has been divided and parity blocks generated from the original blocks, and

a number of the retransmission requests included in the reply message corresponds to the number of required parity blocks for each cluster.

9. The data communication method according to claim 6 , wherein

blocks of the data transmitted or received are clusters including original blocks into which the data has been divided and parity blocks generated from the original blocks, and

a number of the retransmission requests included in the reply message corresponds to the number of required parity blocks for each cluster.