Packet relay apparatus and method of relaying packet

1. A packet relay apparatus configured to relay a packet by utilizing a multi-stage queue structure, comprising:

a receiver configured to receive a packet;

a determiner configured to determine whether to drop the received packet at a latter stage, based on former-stage queue information representing a state of a former-stage queue at any former stage to which the received packet belongs, and latter-stage queue information including a drop probability and representing a state of a latter-stage queue at the latter stage to which the received packet belongs;

a buffer memory configured to hold the received packet on the basis of the determination by the determiner; and

a queuing controller configured to queue the received packet into the former-stage queue or the latter-stage queue in the buffer memory if the determiner determines not to drop the received packet,

wherein when an average queue length is less than a maximum threshold value and greater than a minimum threshold value which is decreased by a drop threshold weight, the drop probability DP is defined as

DP=(MAX_DP+β)( Q AVE−MIN_TH( N+ 1)+β)/(MAX_TH( N+ 1)−MIN_TH( N+ 1)+β),

wherein MAX_DP is a maximum drop probability, α is the drop probability weight, β is the drop threshold weight, QAVE is the average queue length, MAX_TH(N+1) is the maximum threshold value, and MIN_TH(N+1) is the minimum threshold value.

2. The packet relay apparatus according to claim 1 , wherein the determiner is further configured to:

set a relation of the drop probability to the latter-stage queue information based on the former-stage queue information; and

determine whether to drop the received packet, based on the drop probability computed from the latter-stage queue information.

3. The packet relay apparatus according to claim 2 , wherein a constant included in a computational expression used for computing the drop probability is set in response to a user's instruction.

4. The packet relay apparatus according to claim 2 , wherein a computational expression used for computing the drop probability is set in response to a user's instruction.

5. The packet relay apparatus according to claim 1 , wherein the determiner is further configured to:

classify the received packet into one of plural classes based on the former-stage queue information; and

determine whether to drop the received packet based on a threshold value set in each of the plural classes and the latter-stage queue information.

6. The packet relay apparatus according to claim 1 , wherein the former-stage queue information includes a queue length of the queue at any former stage.

7. The packet relay apparatus according to claim 1 , wherein the former-stage queue information includes a communication traffic of the queue at any former stage.

8. A method of relaying a packet by utilizing a multi-stage queue structure, comprising:

(a) receiving a packet;

(b) determining whether to drop the received packet at a latter stage, based on former-stage queue information representing a state of a former-stage queue at any former stage to which the received packet belongs and latter-stage queue information including a drop probability and representing a state of a latter-stage queue at the latter stage to which the received packet belongs;

(c) holding the received packet in a buffer memory on the basis of the determination by the determiner; and

(d) queuing the received packet into the former-stage queue or the latter-stage queue in the buffer memory if it is determined that the received packet will not be dropped,

wherein when an average queue length is less than a maximum threshold value and greater than a minimum threshold value which is decreased by a drop threshold weight, the drop probability DP is defined as

DP=(MAX_DP+α)( Q AVE−MIN_TH( N+ 1)+β)/(MAX_TH( N+ 1)−MIN_TH( N+ 1)+β),

wherein MAX_DP is a maximum drop probability, α is the drop probability weight, β is the drop threshold weight, QAVE is the average queue length, MAX_TH(N+1) is the maximum threshold value, and MIN_TH(N+1) is the minimum threshold value.

9. The method according to claim 8 , further comprising:

setting a relation of the drop probability to the latter-stage queue information based on the former-stage queue information;

wherein determining whether to drop the received packet is further based on the drop probability computed from the latter-stage queue information.

10. The method according to claim 9 , wherein a constant included in a computational expression used for computing the drop probability is set in response to a user's instruction.

11. The method according to claim 9 , wherein a computational expression used for computing the drop probability is set in response to a user's instruction.

12. The method according to claim 8 , further comprising:

classifying the received packet into one of plural classes based on the former-stage queue information;

wherein determining whether to drop the received packet is further based on a threshold value set in each of the plural classes and the latter-stage queue information.

13. The method according to claim 8 , wherein the former-stage queue information includes a queue length of the queue at any former stage.

14. The method according to claim 8 , wherein the former-stage queue information includes a communication traffic of the queue at any former stage.

15. The packet relay apparatus of claim 1 , wherein the drop probability is 100% when the average queue length is greater than or equal to the maximum threshold value, and the drop probability is zero when the average queue length is less than or equal to the minimum threshold value decreased by the drop probability weight.

16. The packet relay apparatus of claim 1 , wherein the drop probability weight is defined as α=αMAX*{QLEN(N)/MAX_TH(N)*2−1} and the drop threshold weight is defined as β=βMAX*{QLEN(N)/MAX_TH(N)*2−1}.

wherein αMAX and βMAX are preset constands, QLEN(N) is a queue length at any stage.